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X11R7.4 - man page for perltrap (x11r4 section 1)

PERLTRAP(1)			 Perl Programmers Reference Guide		      PERLTRAP(1)

NAME
       perltrap - Perl traps for the unwary

DESCRIPTION
       The biggest trap of all is forgetting to "use warnings" or use the -w switch; see per-
       llexwarn and perlrun. The second biggest trap is not making your entire program runnable
       under "use strict".  The third biggest trap is not reading the list of changes in this
       version of Perl; see perldelta.

       Awk Traps

       Accustomed awk users should take special note of the following:

       o   A Perl program executes only once, not once for each input line.  You can do an
	   implicit loop with "-n" or "-p".

       o   The English module, loaded via

	       use English;

	   allows you to refer to special variables (like $/) with names (like $RS), as though
	   they were in awk; see perlvar for details.

       o   Semicolons are required after all simple statements in Perl (except at the end of a
	   block).  Newline is not a statement delimiter.

       o   Curly brackets are required on "if"s and "while"s.

       o   Variables begin with "$", "@" or "%" in Perl.

       o   Arrays index from 0.  Likewise string positions in substr() and index().

       o   You have to decide whether your array has numeric or string indices.

       o   Hash values do not spring into existence upon mere reference.

       o   You have to decide whether you want to use string or numeric comparisons.

       o   Reading an input line does not split it for you.  You get to split it to an array
	   yourself.  And the split() operator has different arguments than awk's.

       o   The current input line is normally in $_, not $0.  It generally does not have the new-
	   line stripped.  ($0 is the name of the program executed.)  See perlvar.

       o   $<digit> does not refer to fields--it refers to substrings matched by the last match
	   pattern.

       o   The print() statement does not add field and record separators unless you set $, and
	   "$\".  You can set $OFS and $ORS if you're using the English module.

       o   You must open your files before you print to them.

       o   The range operator is "..", not comma.  The comma operator works as in C.

       o   The match operator is "=~", not "~".  ("~" is the one's complement operator, as in C.)

       o   The exponentiation operator is "**", not "^".  "^" is the XOR operator, as in C.  (You
	   know, one could get the feeling that awk is basically incompatible with C.)

       o   The concatenation operator is ".", not the null string.  (Using the null string would
	   render "/pat/ /pat/" unparsable, because the third slash would be interpreted as a
	   division operator--the tokenizer is in fact slightly context sensitive for operators
	   like "/", "?", and ">".  And in fact, "." itself can be the beginning of a number.)

       o   The "next", "exit", and "continue" keywords work differently.

       o   The following variables work differently:

		 Awk	   Perl
		 ARGC	   scalar @ARGV (compare with $#ARGV)
		 ARGV[0]   $0
		 FILENAME  $ARGV
		 FNR	   $. - something
		 FS	   (whatever you like)
		 NF	   $#Fld, or some such
		 NR	   $.
		 OFMT	   $#
		 OFS	   $,
		 ORS	   $\
		 RLENGTH   length($&)
		 RS	   $/
		 RSTART    length($`)
		 SUBSEP    $;

       o   You cannot set $RS to a pattern, only a string.

       o   When in doubt, run the awk construct through a2p and see what it gives you.

       C/C++ Traps

       Cerebral C and C++ programmers should take note of the following:

       o   Curly brackets are required on "if"'s and "while"'s.

       o   You must use "elsif" rather than "else if".

       o   The "break" and "continue" keywords from C become in Perl "last" and "next", respec-
	   tively.  Unlike in C, these do not work within a "do { } while" construct.  See "Loop
	   Control" in perlsyn.

       o   There's no switch statement.  (But it's easy to build one on the fly, see "Basic
	   BLOCKs and Switch Statements" in perlsyn)

       o   Variables begin with "$", "@" or "%" in Perl.

       o   Comments begin with "#", not "/*" or "//".  Perl may interpret C/C++ comments as divi-
	   sion operators, unterminated regular expressions or the defined-or operator.

       o   You can't take the address of anything, although a similar operator in Perl is the
	   backslash, which creates a reference.

       o   "ARGV" must be capitalized.	$ARGV[0] is C's "argv[1]", and "argv[0]" ends up in $0.

       o   System calls such as link(), unlink(), rename(), etc. return nonzero for success, not
	   0. (system(), however, returns zero for success.)

       o   Signal handlers deal with signal names, not numbers.  Use "kill -l" to find their
	   names on your system.

       Sed Traps

       Seasoned sed programmers should take note of the following:

       o   A Perl program executes only once, not once for each input line.  You can do an
	   implicit loop with "-n" or "-p".

       o   Backreferences in substitutions use "$" rather than "\".

       o   The pattern matching metacharacters "(", ")", and "|" do not have backslashes in
	   front.

       o   The range operator is "...", rather than comma.

       Shell Traps

       Sharp shell programmers should take note of the following:

       o   The backtick operator does variable interpolation without regard to the presence of
	   single quotes in the command.

       o   The backtick operator does no translation of the return value, unlike csh.

       o   Shells (especially csh) do several levels of substitution on each command line.  Perl
	   does substitution in only certain constructs such as double quotes, backticks, angle
	   brackets, and search patterns.

       o   Shells interpret scripts a little bit at a time.  Perl compiles the entire program
	   before executing it (except for "BEGIN" blocks, which execute at compile time).

       o   The arguments are available via @ARGV, not $1, $2, etc.

       o   The environment is not automatically made available as separate scalar variables.

       o   The shell's "test" uses "=", "!=", "<" etc for string comparisons and "-eq", "-ne",
	   "-lt" etc for numeric comparisons. This is the reverse of Perl, which uses "eq", "ne",
	   "lt" for string comparisons, and "==", "!=" "<" etc for numeric comparisons.

       Perl Traps

       Practicing Perl Programmers should take note of the following:

       o   Remember that many operations behave differently in a list context than they do in a
	   scalar one.	See perldata for details.

       o   Avoid barewords if you can, especially all lowercase ones.  You can't tell by just
	   looking at it whether a bareword is a function or a string.	By using quotes on
	   strings and parentheses on function calls, you won't ever get them confused.

       o   You cannot discern from mere inspection which builtins are unary operators (like
	   chop() and chdir()) and which are list operators (like print() and unlink()).  (Unless
	   prototyped, user-defined subroutines can only be list operators, never unary ones.)
	   See perlop and perlsub.

       o   People have a hard time remembering that some functions default to $_, or @ARGV, or
	   whatever, but that others which you might expect to do not.

       o   The <FH> construct is not the name of the filehandle, it is a readline operation on
	   that handle.  The data read is assigned to $_ only if the file read is the sole condi-
	   tion in a while loop:

	       while (<FH>)	 { }
	       while (defined($_ = <FH>)) { }..
	       <FH>;  # data discarded!

       o   Remember not to use "=" when you need "=~"; these two constructs are quite different:

	       $x =  /foo/;
	       $x =~ /foo/;

       o   The "do {}" construct isn't a real loop that you can use loop control on.

       o   Use "my()" for local variables whenever you can get away with it (but see perlform for
	   where you can't).  Using "local()" actually gives a local value to a global variable,
	   which leaves you open to unforeseen side-effects of dynamic scoping.

       o   If you localize an exported variable in a module, its exported value will not change.
	   The local name becomes an alias to a new value but the external name is still an alias
	   for the original.

       Perl4 to Perl5 Traps

       Practicing Perl4 Programmers should take note of the following Perl4-to-Perl5 specific
       traps.

       They're crudely ordered according to the following list:

       Discontinuance, Deprecation, and BugFix traps
	   Anything that's been fixed as a perl4 bug, removed as a perl4 feature or deprecated as
	   a perl4 feature with the intent to encourage usage of some other perl5 feature.

       Parsing Traps
	   Traps that appear to stem from the new parser.

       Numerical Traps
	   Traps having to do with numerical or mathematical operators.

       General data type traps
	   Traps involving perl standard data types.

       Context Traps - scalar, list contexts
	   Traps related to context within lists, scalar statements/declarations.

       Precedence Traps
	   Traps related to the precedence of parsing, evaluation, and execution of code.

       General Regular Expression Traps using s///, etc.
	   Traps related to the use of pattern matching.

       Subroutine, Signal, Sorting Traps
	   Traps related to the use of signals and signal handlers, general subroutines, and
	   sorting, along with sorting subroutines.

       OS Traps
	   OS-specific traps.

       DBM Traps
	   Traps specific to the use of "dbmopen()", and specific dbm implementations.

       Unclassified Traps
	   Everything else.

       If you find an example of a conversion trap that is not listed here, please submit it to
       <perlbug@perl.org> for inclusion.  Also note that at least some of these can be caught
       with the "use warnings" pragma or the -w switch.

       Discontinuance, Deprecation, and BugFix traps

       Anything that has been discontinued, deprecated, or fixed as a bug from perl4.

       * Symbols starting with "_" no longer forced into main
	   Symbols starting with "_" are no longer forced into package main, except for $_ itself
	   (and @_, etc.).

	       package test;
	       $_legacy = 1;

	       package main;
	       print "\$_legacy is ",$_legacy,"\n";

	       # perl4 prints: $_legacy is 1
	       # perl5 prints: $_legacy is

       * Double-colon valid package separator in variable name
	   Double-colon is now a valid package separator in a variable name.  Thus these behave
	   differently in perl4 vs. perl5, because the packages don't exist.

	       $a=1;$b=2;$c=3;$var=4;
	       print "$a::$b::$c ";
	       print "$var::abc::xyz\n";

	       # perl4 prints: 1::2::3 4::abc::xyz
	       # perl5 prints: 3

	   Given that "::" is now the preferred package delimiter, it is debatable whether this
	   should be classed as a bug or not.  (The older package delimiter, ' ,is used here)

	       $x = 10;
	       print "x=${'x}\n";

	       # perl4 prints: x=10
	       # perl5 prints: Can't find string terminator "'" anywhere before EOF

	   You can avoid this problem, and remain compatible with perl4, if you always explicitly
	   include the package name:

	       $x = 10;
	       print "x=${main'x}\n";

	   Also see precedence traps, for parsing $:.

       * 2nd and 3rd args to "splice()" are now in scalar context
	   The second and third arguments of "splice()" are now evaluated in scalar context (as
	   the Camel says) rather than list context.

	       sub sub1{return(0,2) }	       # return a 2-element list
	       sub sub2{ return(1,2,3)}        # return a 3-element list
	       @a1 = ("a","b","c","d","e");
	       @a2 = splice(@a1,&sub1,&sub2);
	       print join(' ',@a2),"\n";

	       # perl4 prints: a b
	       # perl5 prints: c d e

       * Can't do "goto" into a block that is optimized away
	   You can't do a "goto" into a block that is optimized away.  Darn.

	       goto marker1;

	       for(1){
	       marker1:
		   print "Here I is!\n";
	       }

	       # perl4 prints: Here I is!
	       # perl5 errors: Can't "goto" into the middle of a foreach loop

       * Can't use whitespace as variable name or quote delimiter
	   It is no longer syntactically legal to use whitespace as the name of a variable, or as
	   a delimiter for any kind of quote construct.  Double darn.

	       $a = ("foo bar");
	       $b = q baz;
	       print "a is $a, b is $b\n";

	       # perl4 prints: a is foo bar, b is baz
	       # perl5 errors: Bareword found where operator expected

       * "while/if BLOCK BLOCK" gone
	   The archaic while/if BLOCK BLOCK syntax is no longer supported.

	       if { 1 } {
		   print "True!";
	       }
	       else {
		   print "False!";
	       }

	       # perl4 prints: True!
	       # perl5 errors: syntax error at test.pl line 1, near "if {"

       * "**" binds tighter than unary minus
	   The "**" operator now binds more tightly than unary minus.  It was documented to work
	   this way before, but didn't.

	       print -4**2,"\n";

	       # perl4 prints: 16
	       # perl5 prints: -16

       * "foreach" changed when iterating over a list
	   The meaning of "foreach{}" has changed slightly when it is iterating over a list which
	   is not an array.  This used to assign the list to a temporary array, but no longer
	   does so (for efficiency).  This means that you'll now be iterating over the actual
	   values, not over copies of the values.  Modifications to the loop variable can change
	   the original values.

	       @list = ('ab','abc','bcd','def');
	       foreach $var (grep(/ab/,@list)){
		   $var = 1;
	       }
	       print (join(':',@list));

	       # perl4 prints: ab:abc:bcd:def
	       # perl5 prints: 1:1:bcd:def

	   To retain Perl4 semantics you need to assign your list explicitly to a temporary array
	   and then iterate over that.	For example, you might need to change

	       foreach $var (grep(/ab/,@list)){

	   to

	       foreach $var (@tmp = grep(/ab/,@list)){

	   Otherwise changing $var will clobber the values of @list.  (This most often happens
	   when you use $_ for the loop variable, and call subroutines in the loop that don't
	   properly localize $_.)

       * "split" with no args behavior changed
	   "split" with no arguments now behaves like "split ' '" (which doesn't return an ini-
	   tial null field if $_ starts with whitespace), it used to behave like "split /\s+/"
	   (which does).

	       $_ = ' hi mom';
	       print join(':', split);

	       # perl4 prints: :hi:mom
	       # perl5 prints: hi:mom

       * -e behavior fixed
	   Perl 4 would ignore any text which was attached to an -e switch, always taking the
	   code snippet from the following arg.  Additionally, it would silently accept an -e
	   switch without a following arg.  Both of these behaviors have been fixed.

	       perl -e'print "attached to -e"' 'print "separate arg"'

	       # perl4 prints: separate arg
	       # perl5 prints: attached to -e

	       perl -e

	       # perl4 prints:
	       # perl5 dies: No code specified for -e.

       * "push" returns number of elements in resulting list
	   In Perl 4 the return value of "push" was undocumented, but it was actually the last
	   value being pushed onto the target list.  In Perl 5 the return value of "push" is doc-
	   umented, but has changed, it is the number of elements in the resulting list.

	       @x = ('existing');
	       print push(@x, 'first new', 'second new');

	       # perl4 prints: second new
	       # perl5 prints: 3

       * Some error messages differ
	   Some error messages will be different.

       * "split()" honors subroutine args
	   In Perl 4, if in list context the delimiters to the first argument of "split()" were
	   "??", the result would be placed in @_ as well as being returned.   Perl 5 has more
	   respect for your subroutine arguments.

       * Bugs removed
	   Some bugs may have been inadvertently removed.  :-)

       Parsing Traps

       Perl4-to-Perl5 traps from having to do with parsing.

       * Space between . and = triggers syntax error
	   Note the space between . and =

	       $string . = "more string";
	       print $string;

	       # perl4 prints: more string
	       # perl5 prints: syntax error at - line 1, near ". ="

       * Better parsing in perl 5
	   Better parsing in perl 5

	       sub foo {}
	       &foo
	       print("hello, world\n");

	       # perl4 prints: hello, world
	       # perl5 prints: syntax error

       * Function parsing
	   "if it looks like a function, it is a function" rule.

	     print
	       ($foo == 1) ? "is one\n" : "is zero\n";

	       # perl4 prints: is zero
	       # perl5 warns: "Useless use of a constant in void context" if using -w

       * String interpolation of $#array differs
	   String interpolation of the $#array construct differs when braces are to used around
	   the name.

	       @a = (1..3);
	       print "${#a}";

	       # perl4 prints: 2
	       # perl5 fails with syntax error

	       @ = (1..3);
	       print "$#{a}";

	       # perl4 prints: {a}
	       # perl5 prints: 2

       * Perl guesses on "map", "grep" followed by "{" if it starts BLOCK or hash ref
	   When perl sees "map {" (or "grep {"), it has to guess whether the "{" starts a BLOCK
	   or a hash reference. If it guesses wrong, it will report a syntax error near the "}"
	   and the missing (or unexpected) comma.

	   Use unary "+" before "{" on a hash reference, and unary "+" applied to the first thing
	   in a BLOCK (after "{"), for perl to guess right all the time. (See "map" in perlfunc.)

       Numerical Traps

       Perl4-to-Perl5 traps having to do with numerical operators, operands, or output from same.

       * Formatted output and significant digits
	    Formatted output and significant digits.  In general, Perl 5 tries to be more pre-
	    cise.  For example, on a Solaris Sparc:

		print 7.373504 - 0, "\n";
		printf "%20.18f\n", 7.373504 - 0;

		# Perl4 prints:
		7.3750399999999996141
		7.375039999999999614

		# Perl5 prints:
		7.373504
		7.375039999999999614

	    Notice how the first result looks better in Perl 5.

	    Your results may vary, since your floating point formatting routines and even float-
	    ing point format may be slightly different.

       * Auto-increment operator over signed int limit deleted
	    This specific item has been deleted.  It demonstrated how the auto-increment operator
	    would not catch when a number went over the signed int limit.  Fixed in version
	    5.003_04.  But always be wary when using large integers.  If in doubt:

	       use Math::BigInt;

       * Assignment of return values from numeric equality tests doesn't work
	    Assignment of return values from numeric equality tests does not work in perl5 when
	    the test evaluates to false(0).  Logical tests now return a null, instead of 0

		$p = ($test == 1);
		print $p,"\n";

		# perl4 prints: 0
		# perl5 prints:

	    Also see "General Regular Expression Traps using s///, etc."  for another example of
	    this new feature...

       * Bitwise string ops
	    When bitwise operators which can operate upon either numbers or strings ("& | ^ ~")
	    are given only strings as arguments, perl4 would treat the operands as bitstrings so
	    long as the program contained a call to the "vec()" function. perl5 treats the string
	    operands as bitstrings.  (See "Bitwise String Operators" in perlop for more details.)

		$fred = "10";
		$barney = "12";
		$betty = $fred & $barney;
		print "$betty\n";
		# Uncomment the next line to change perl4's behavior
		# ($dummy) = vec("dummy", 0, 0);

		# Perl4 prints:
		8

		# Perl5 prints:
		10

		# If vec() is used anywhere in the program, both print:
		10

       General data type traps

       Perl4-to-Perl5 traps involving most data-types, and their usage within certain expressions
       and/or context.

       * Negative array subscripts now count from the end of array
	    Negative array subscripts now count from the end of the array.

		@a = (1, 2, 3, 4, 5);
		print "The third element of the array is $a[3] also expressed as $a[-2] \n";

		# perl4 prints: The third element of the array is 4 also expressed as
		# perl5 prints: The third element of the array is 4 also expressed as 4

       * Setting $#array lower now discards array elements
	    Setting $#array lower now discards array elements, and makes them impossible to
	    recover.

		@a = (a,b,c,d,e);
		print "Before: ",join('',@a);
		$#a =1;
		print ", After: ",join('',@a);
		$#a =3;
		print ", Recovered: ",join('',@a),"\n";

		# perl4 prints: Before: abcde, After: ab, Recovered: abcd
		# perl5 prints: Before: abcde, After: ab, Recovered: ab

       * Hashes get defined before use
	    Hashes get defined before use

		local($s,@a,%h);
		die "scalar \$s defined" if defined($s);
		die "array \@a defined" if defined(@a);
		die "hash \%h defined" if defined(%h);

		# perl4 prints:
		# perl5 dies: hash %h defined

	    Perl will now generate a warning when it sees defined(@a) and defined(%h).

       * Glob assignment from localized variable to variable
	    glob assignment from variable to variable will fail if the assigned variable is
	    localized subsequent to the assignment

		@a = ("This is Perl 4");
		*b = *a;
		local(@a);
		print @b,"\n";

		# perl4 prints: This is Perl 4
		# perl5 prints:

       * Assigning "undef" to glob
	    Assigning "undef" to a glob has no effect in Perl 5.   In Perl 4 it undefines the
	    associated scalar (but may have other side effects including SEGVs). Perl 5 will also
	    warn if "undef" is assigned to a typeglob. (Note that assigning "undef" to a typeglob
	    is different than calling the "undef" function on a typeglob ("undef *foo"), which
	    has quite a few effects.

		$foo = "bar";
		*foo = undef;
		print $foo;

		# perl4 prints:
		# perl4 warns: "Use of uninitialized variable" if using -w
		# perl5 prints: bar
		# perl5 warns: "Undefined value assigned to typeglob" if using -w

       * Changes in unary negation (of strings)
	    Changes in unary negation (of strings) This change effects both the return value and
	    what it does to auto(magic)increment.

		$x = "aaa";
		print ++$x," : ";
		print -$x," : ";
		print ++$x,"\n";

		# perl4 prints: aab : -0 : 1
		# perl5 prints: aab : -aab : aac

       * Modifying of constants prohibited
	    perl 4 lets you modify constants:

		$foo = "x";
		&mod($foo);
		for ($x = 0; $x < 3; $x++) {
		    &mod("a");
		}
		sub mod {
		    print "before: $_[0]";
		    $_[0] = "m";
		    print "  after: $_[0]\n";
		}

		# perl4:
		# before: x  after: m
		# before: a  after: m
		# before: m  after: m
		# before: m  after: m

		# Perl5:
		# before: x  after: m
		# Modification of a read-only value attempted at foo.pl line 12.
		# before: a

       * "defined $var" behavior changed
	    The behavior is slightly different for:

		print "$x", defined $x

		# perl 4: 1
		# perl 5: <no output, $x is not called into existence>

       * Variable Suicide
	    Variable suicide behavior is more consistent under Perl 5.	Perl5 exhibits the same
	    behavior for hashes and scalars, that perl4 exhibits for only scalars.

		$aGlobal{ "aKey" } = "global value";
		print "MAIN:", $aGlobal{"aKey"}, "\n";
		$GlobalLevel = 0;
		&test( *aGlobal );

		sub test {
		    local( *theArgument ) = @_;
		    local( %aNewLocal ); # perl 4 != 5.001l,m
		    $aNewLocal{"aKey"} = "this should never appear";
		    print "SUB: ", $theArgument{"aKey"}, "\n";
		    $aNewLocal{"aKey"} = "level $GlobalLevel";	 # what should print
		    $GlobalLevel++;
		    if( $GlobalLevel<4 ) {
			&test( *aNewLocal );
		    }
		}

		# Perl4:
		# MAIN:global value
		# SUB: global value
		# SUB: level 0
		# SUB: level 1
		# SUB: level 2

		# Perl5:
		# MAIN:global value
		# SUB: global value
		# SUB: this should never appear
		# SUB: this should never appear
		# SUB: this should never appear

       Context Traps - scalar, list contexts

       * Elements of argument lists for formats evaluated in list context
	    The elements of argument lists for formats are now evaluated in list context.  This
	    means you can interpolate list values now.

		@fmt = ("foo","bar","baz");
		format STDOUT=
		@<<<<< @||||| @>>>>>
		@fmt;
		.
		write;

		# perl4 errors:  Please use commas to separate fields in file
		# perl5 prints: foo	bar	 baz

       * "caller()" returns false value in scalar context if no caller present
	    The "caller()" function now returns a false value in a scalar context if there is no
	    caller.  This lets library files determine if they're being required.

		caller() ? (print "You rang?\n") : (print "Got a 0\n");

		# perl4 errors: There is no caller
		# perl5 prints: Got a 0

       * Comma operator in scalar context gives scalar context to args
	    The comma operator in a scalar context is now guaranteed to give a scalar context to
	    its arguments.

		@y= ('a','b','c');
		$x = (1, 2, @y);
		print "x = $x\n";

		# Perl4 prints:  x = c	 # Thinks list context interpolates list
		# Perl5 prints:  x = 3	 # Knows scalar uses length of list

       * "sprintf()" prototyped as "($;@)"
	    "sprintf()" is prototyped as ($;@), so its first argument is given scalar context.
	    Thus, if passed an array, it will probably not do what you want, unlike Perl 4:

		@z = ('%s%s', 'foo', 'bar');
		$x = sprintf(@z);
		print $x;

		# perl4 prints: foobar
		# perl5 prints: 3

	    "printf()" works the same as it did in Perl 4, though:

		@z = ('%s%s', 'foo', 'bar');
		printf STDOUT (@z);

		# perl4 prints: foobar
		# perl5 prints: foobar

       Precedence Traps

       Perl4-to-Perl5 traps involving precedence order.

       Perl 4 has almost the same precedence rules as Perl 5 for the operators that they both
       have.  Perl 4 however, seems to have had some inconsistencies that made the behavior dif-
       fer from what was documented.

       * LHS vs. RHS of any assignment operator
	    LHS vs. RHS of any assignment operator.  LHS is evaluated first in perl4, second in
	    perl5; this can affect the relationship between side-effects in sub-expressions.

		@arr = ( 'left', 'right' );
		$a{shift @arr} = shift @arr;
		print join( ' ', keys %a );

		# perl4 prints: left
		# perl5 prints: right

       * Semantic errors introduced due to precedence
	    These are now semantic errors because of precedence:

		@list = (1,2,3,4,5);
		%map = ("a",1,"b",2,"c",3,"d",4);
		$n = shift @list + 2;	# first item in list plus 2
		print "n is $n, ";
		$m = keys %map + 2;	# number of items in hash plus 2
		print "m is $m\n";

		# perl4 prints: n is 3, m is 6
		# perl5 errors and fails to compile

       * Precedence of assignment operators same as the precedence of assignment
	    The precedence of assignment operators is now the same as the precedence of assign-
	    ment.  Perl 4 mistakenly gave them the precedence of the associated operator.  So you
	    now must parenthesize them in expressions like

		/foo/ ? ($a += 2) : ($a -= 2);

	    Otherwise

		/foo/ ? $a += 2 : $a -= 2

	    would be erroneously parsed as

		(/foo/ ? $a += 2 : $a) -= 2;

	    On the other hand,

		$a += /foo/ ? 1 : 2;

	    now works as a C programmer would expect.

       * "open" requires parentheses around filehandle
		open FOO || die;

	    is now incorrect.  You need parentheses around the filehandle.  Otherwise, perl5
	    leaves the statement as its default precedence:

		open(FOO || die);

		# perl4 opens or dies
		# perl5 opens FOO, dying only if 'FOO' is false, i.e. never

       * $: precedence over $:: gone
	    perl4 gives the special variable, $: precedence, where perl5 treats $:: as main
	    "package"

		$a = "x"; print "$::a";

		# perl 4 prints: -:a
		# perl 5 prints: x

       * Precedence of file test operators documented
	    perl4 had buggy precedence for the file test operators vis-a-vis the assignment oper-
	    ators.  Thus, although the precedence table for perl4 leads one to believe "-e $foo
	    .= "q"" should parse as "((-e $foo) .= "q")", it actually parses as "(-e ($foo .=
	    "q"))".  In perl5, the precedence is as documented.

		-e $foo .= "q"

		# perl4 prints: no output
		# perl5 prints: Can't modify -e in concatenation

       * "keys", "each", "values" are regular named unary operators
	    In perl4, keys(), each() and values() were special high-precedence operators that
	    operated on a single hash, but in perl5, they are regular named unary operators.  As
	    documented, named unary operators have lower precedence than the arithmetic and con-
	    catenation operators "+ - .", but the perl4 variants of these operators actually bind
	    tighter than "+ - .".  Thus, for:

		%foo = 1..10;
		print keys %foo - 1

		# perl4 prints: 4
		# perl5 prints: Type of arg 1 to keys must be hash (not subtraction)

	    The perl4 behavior was probably more useful, if less consistent.

       General Regular Expression Traps using s///, etc.

       All types of RE traps.

       * "s'$lhs'$rhs'" interpolates on either side
	    "s'$lhs'$rhs'" now does no interpolation on either side.  It used to interpolate $lhs
	    but not $rhs.  (And still does not match a literal '$' in string)

		$a=1;$b=2;
		$string = '1 2 $a $b';
		$string =~ s'$a'$b';
		print $string,"\n";

		# perl4 prints: $b 2 $a $b
		# perl5 prints: 1 2 $a $b

       * "m//g" attaches its state to the searched string
	    "m//g" now attaches its state to the searched string rather than the regular expres-
	    sion.  (Once the scope of a block is left for the sub, the state of the searched
	    string is lost)

		$_ = "ababab";
		while(m/ab/g){
		    &doit("blah");
		}
		sub doit{local($_) = shift; print "Got $_ "}

		# perl4 prints: Got blah Got blah Got blah Got blah
		# perl5 prints: infinite loop blah...

       * "m//o" used within an anonymous sub
	    Currently, if you use the "m//o" qualifier on a regular expression within an anony-
	    mous sub, all closures generated from that anonymous sub will use the regular expres-
	    sion as it was compiled when it was used the very first time in any such closure.
	    For instance, if you say

		sub build_match {
		    my($left,$right) = @_;
		    return sub { $_[0] =~ /$left stuff $right/o; };
		}
		$good = build_match('foo','bar');
		$bad = build_match('baz','blarch');
		print $good->('foo stuff bar') ? "ok\n" : "not ok\n";
		print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n";
		print $bad->('foo stuff bar') ? "not ok\n" : "ok\n";

	    For most builds of Perl5, this will print: ok not ok not ok

	    build_match() will always return a sub which matches the contents of $left and $right
	    as they were the first time that build_match() was called, not as they are in the
	    current call.

       * $+ isn't set to whole match
	    If no parentheses are used in a match, Perl4 sets $+ to the whole match, just like
	    $&. Perl5 does not.

		"abcdef" =~ /b.*e/;
		print "\$+ = $+\n";

		# perl4 prints: bcde
		# perl5 prints:

       * Substitution now returns null string if it fails
	    substitution now returns the null string if it fails

		$string = "test";
		$value = ($string =~ s/foo//);
		print $value, "\n";

		# perl4 prints: 0
		# perl5 prints:

	    Also see "Numerical Traps" for another example of this new feature.

       * "s`lhs`rhs`" is now a normal substitution
	    "s`lhs`rhs`" (using backticks) is now a normal substitution, with no backtick expan-
	    sion

		$string = "";
		$string =~ s`^`hostname`;
		print $string, "\n";

		# perl4 prints: <the local hostname>
		# perl5 prints: hostname

       * Stricter parsing of variables in regular expressions
	    Stricter parsing of variables used in regular expressions

		s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o;

		# perl4: compiles w/o error
		# perl5: with Scalar found where operator expected ..., near "$opt$plus"

	    an added component of this example, apparently from the same script, is the actual
	    value of the s'd string after the substitution.  "[$opt]" is a character class in
	    perl4 and an array subscript in perl5

		$grpc = 'a';
		$opt  = 'r';
		$_ = 'bar';
		s/^([^$grpc]*$grpc[$opt]?)/foo/;
		print;

		# perl4 prints: foo
		# perl5 prints: foobar

       * "m?x?" matches only once
	    Under perl5, "m?x?" matches only once, like "?x?". Under perl4, it matched repeat-
	    edly, like "/x/" or "m!x!".

		$test = "once";
		sub match { $test =~ m?once?; }
		&match();
		if( &match() ) {
		    # m?x? matches more then once
		    print "perl4\n";
		} else {
		    # m?x? matches only once
		    print "perl5\n";
		}

		# perl4 prints: perl4
		# perl5 prints: perl5

       * Failed matches don't reset the match variables
	    Unlike in Ruby, failed matches in Perl do not reset the match variables ($1, $2, ...,
	    $`, ...).

       Subroutine, Signal, Sorting Traps

       The general group of Perl4-to-Perl5 traps having to do with Signals, Sorting, and their
       related subroutines, as well as general subroutine traps.  Includes some OS-Specific
       traps.

       * Barewords that used to look like strings look like subroutine calls
	    Barewords that used to look like strings to Perl will now look like subroutine calls
	    if a subroutine by that name is defined before the compiler sees them.

		sub SeeYa { warn"Hasta la vista, baby!" }
		$SIG{'TERM'} = SeeYa;
		print "SIGTERM is now $SIG{'TERM'}\n";

		# perl4 prints: SIGTERM is now main'SeeYa
		# perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!")

	    Use -w to catch this one

       * Reverse is no longer allowed as the name of a sort subroutine
	    reverse is no longer allowed as the name of a sort subroutine.

		sub reverse{ print "yup "; $a <=> $b }
		print sort reverse (2,1,3);

		# perl4 prints: yup yup 123
		# perl5 prints: 123
		# perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse()

       * "warn()" won't let you specify a filehandle.
	    Although it _always_ printed to STDERR, warn() would let you specify a filehandle in
	    perl4.  With perl5 it does not.

		warn STDERR "Foo!";

		# perl4 prints: Foo!
		# perl5 prints: String found where operator expected

       OS Traps

       * SysV resets signal handler correctly
	    Under HPUX, and some other SysV OSes, one had to reset any signal handler, within
	    the signal handler function, each time a signal was handled with perl4.  With perl5,
	    the reset is now done correctly.  Any code relying on the handler _not_ being reset
	    will have to be reworked.

	    Since version 5.002, Perl uses sigaction() under SysV.

		sub gotit {
		    print "Got @_... ";
		}
		$SIG{'INT'} = 'gotit';

		$| = 1;
		$pid = fork;
		if ($pid) {
		    kill('INT', $pid);
		    sleep(1);
		    kill('INT', $pid);
		} else {
		    while(1) {sleep(10);}
		}

		# perl4 (HPUX) prints: Got INT...
		# perl5 (HPUX) prints: Got INT... Got INT...

       * SysV "seek()" appends correctly
	    Under SysV OSes, "seek()" on a file opened to append ">>" now does the right thing
	    w.r.t. the fopen() manpage. e.g., - When a file is opened for append,  it  is  impos-
	    sible to overwrite information already in the file.

		open(TEST,">>seek.test");
		$start = tell TEST;
		foreach(1 .. 9){
		    print TEST "$_ ";
		}
		$end = tell TEST;
		seek(TEST,$start,0);
		print TEST "18 characters here";

		# perl4 (solaris) seek.test has: 18 characters here
		# perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here

       Interpolation Traps

       Perl4-to-Perl5 traps having to do with how things get interpolated within certain expres-
       sions, statements, contexts, or whatever.

       * "@" always interpolates an array in double-quotish strings
	    @ now always interpolates an array in double-quotish strings.

		print "To: someone@somewhere.com\n";

		# perl4 prints: To:someone@somewhere.com
		# perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere
		# perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string

       * Double-quoted strings may no longer end with an unescaped $
	    Double-quoted strings may no longer end with an unescaped $.

		$foo = "foo$";
		print "foo is $foo\n";

		# perl4 prints: foo is foo$
		# perl5 errors: Final $ should be \$ or $name

	    Note: perl5 DOES NOT error on the terminating @ in $bar

       * Arbitrary expressions are evaluated inside braces within double quotes
	    Perl now sometimes evaluates arbitrary expressions inside braces that occur within
	    double quotes (usually when the opening brace is preceded by "$" or "@").

		@www = "buz";
		$foo = "foo";
		$bar = "bar";
		sub foo { return "bar" };
		print "|@{w.w.w}|${main'foo}|";

		# perl4 prints: |@{w.w.w}|foo|
		# perl5 prints: |buz|bar|

	    Note that you can "use strict;" to ward off such trappiness under perl5.

       * $$x now tries to dereference $x
	    The construct "this is $$x" used to interpolate the pid at that point, but now tries
	    to dereference $x.	$$ by itself still works fine, however.

		$s = "a reference";
		$x = *s;
		print "this is $$x\n";

		# perl4 prints: this is XXXx   (XXX is the current pid)
		# perl5 prints: this is a reference

       * Creation of hashes on the fly with "eval "EXPR"" requires protection
	    Creation of hashes on the fly with "eval "EXPR"" now requires either both "$"'s to be
	    protected in the specification of the hash name, or both curlies to be protected.  If
	    both curlies are protected, the result will be compatible with perl4 and perl5.  This
	    is a very common practice, and should be changed to use the block form of "eval{}"
	    if possible.

		$hashname = "foobar";
		$key = "baz";
		$value = 1234;
		eval "\$$hashname{'$key'} = q|$value|";
		(defined($foobar{'baz'})) ?  (print "Yup") : (print "Nope");

		# perl4 prints: Yup
		# perl5 prints: Nope

	    Changing

		eval "\$$hashname{'$key'} = q|$value|";

	    to

		eval "\$\$hashname{'$key'} = q|$value|";

	    causes the following result:

		# perl4 prints: Nope
		# perl5 prints: Yup

	    or, changing to

		eval "\$$hashname\{'$key'\} = q|$value|";

	    causes the following result:

		# perl4 prints: Yup
		# perl5 prints: Yup
		# and is compatible for both versions

       * Bugs in earlier perl versions
	    perl4 programs which unconsciously rely on the bugs in earlier perl versions.

		perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"'

		# perl4 prints: This is not perl5
		# perl5 prints: This is perl5

       * Array and hash brackets during interpolation
	    You also have to be careful about array and hash brackets during interpolation.

		print "$foo["

		perl 4 prints: [
		perl 5 prints: syntax error

		print "$foo{"

		perl 4 prints: {
		perl 5 prints: syntax error

	    Perl 5 is expecting to find an index or key name following the respective brackets,
	    as well as an ending bracket of the appropriate type.  In order to mimic the behavior
	    of Perl 4, you must escape the bracket like so.

		print "$foo\[";
		print "$foo\{";

       * Interpolation of "\$$foo{bar}"
	    Similarly, watch out for: "\$$foo{bar}"

		$foo = "baz";
		print "\$$foo{bar}\n";

		# perl4 prints: $baz{bar}
		# perl5 prints: $

	    Perl 5 is looking for $foo{bar} which doesn't exist, but perl 4 is happy just to
	    expand $foo to "baz" by itself.  Watch out for this especially in "eval"'s.

       * "qq()" string passed to "eval" will not find string terminator
	    "qq()" string passed to "eval"

		eval qq(
		    foreach \$y (keys %\$x\) {
			\$count++;
		    }
		);

		# perl4 runs this ok
		# perl5 prints: Can't find string terminator ")"

       DBM Traps

       General DBM traps.

       * Perl5 must have been linked with same dbm/ndbm as the default for "dbmopen()"
	    Existing dbm databases created under perl4 (or any other dbm/ndbm tool) may cause the
	    same script, run under perl5, to fail.  The build of perl5 must have been linked with
	    the same dbm/ndbm as the default for "dbmopen()" to function properly without
	    "tie"'ing to an extension dbm implementation.

		dbmopen (%dbm, "file", undef);
		print "ok\n";

		# perl4 prints: ok
		# perl5 prints: ok (IFF linked with -ldbm or -lndbm)

       * DBM exceeding limit on the key/value size will cause perl5 to exit immediately
	    Existing dbm databases created under perl4 (or any other dbm/ndbm tool) may cause the
	    same script, run under perl5, to fail.  The error generated when exceeding the limit
	    on the key/value size will cause perl5 to exit immediately.

		dbmopen(DB, "testdb",0600) || die "couldn't open db! $!";
		$DB{'trap'} = "x" x 1024;  # value too large for most dbm/ndbm
		print "YUP\n";

		# perl4 prints:
		dbm store returned -1, errno 28, key "trap" at - line 3.
		YUP

		# perl5 prints:
		dbm store returned -1, errno 28, key "trap" at - line 3.

       Unclassified Traps

       Everything else.

       * "require"/"do" trap using returned value
	    If the file doit.pl has:

		sub foo {
		    $rc = do "./do.pl";
		    return 8;
		}
		print &foo, "\n";

	    And the do.pl file has the following single line:

		return 3;

	    Running doit.pl gives the following:

		# perl 4 prints: 3 (aborts the subroutine early)
		# perl 5 prints: 8

	    Same behavior if you replace "do" with "require".

       * "split" on empty string with LIMIT specified
		$string = '';
		@list = split(/foo/, $string, 2)

	    Perl4 returns a one element list containing the empty string but Perl5 returns an
	    empty list.

       As always, if any of these are ever officially declared as bugs, they'll be fixed and
       removed.

perl v5.8.9				    2007-11-17				      PERLTRAP(1)


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