#perl -w
$pattern = "file*";                         # process only those files that match pattern "file*"
while (defined ($in = glob($pattern))) {
  ($out = $in) =~ s/\.in$/.out/;            # read from "xyz.in" and write to "xyz.out"
  open (IN, "<", $in) or die "Can't open $in for reading: $!";
  open (OUT,">>", $out) or die "Can't open $out for writing: $!";
  while (<IN>) {
    print OUT $_ if $.%3 == 0;
  }
  close (IN) or die "Can't close $in: $!";  # good idea to do some housekeeping
  close (OUT) or die "Can't close $out: $!";
}

C:\>
C:\>
C:\>rem Display the contents of input files "file*.in"
 
C:\>type file1.in
this is line   1 in file1.in ...
this is line   2 in file1.in ...
this is line   3 in file1.in ...
this is line   4 in file1.in ...
this is line   5 in file1.in ...
this is line   6 in file1.in ...
this is line   7 in file1.in ...
this is line   8 in file1.in ...
this is line   9 in file1.in ...
this is line  10 in file1.in ...
 
C:\>type file2.in
this is line   1 in file2.in ...
this is line   2 in file2.in ...
this is line   3 in file2.in ...
this is line   4 in file2.in ...
this is line   5 in file2.in ...
this is line   6 in file2.in ...
this is line   7 in file2.in ...
this is line   8 in file2.in ...
this is line   9 in file2.in ...
this is line  10 in file2.in ...
 
C:\>type file3.in
this is line   1 in file3.in ...
this is line   2 in file3.in ...
this is line   3 in file3.in ...
this is line   4 in file3.in ...
this is line   5 in file3.in ...
this is line   6 in file3.in ...
this is line   7 in file3.in ...
this is line   8 in file3.in ...
this is line   9 in file3.in ...
this is line  10 in file3.in ...
 
C:\>rem Now display the content of the Perl program
 
C:\>type process_files.pl
#perl -w
$pattern = "file*";                         # process only those files that match pattern "file*"
while (defined ($in = glob($pattern))) {
  ($out = $in) =~ s/\.in$/.out/;            # read from "xyz.in" and write to "xyz.out"
  open (IN, "<", $in) or die "Can't open $in for reading: $!";
  open (OUT,">>", $out) or die "Can't open $out for writing: $!";
  while (<IN>) {
    print OUT $_ if $.%3 == 0;
  }
  close (IN) or die "Can't close $in: $!";  # good idea to do some housekeeping
  close (OUT) or die "Can't close $out: $!";
}
 
C:\>
C:\>rem Run the Perl program
 
C:\>perl process_files.pl
 
C:\>
C:\>rem Now check the output files
 
C:\>type file1.out
this is line   3 in file1.in ...
this is line   6 in file1.in ...
this is line   9 in file1.in ...
 
C:\>type file2.out
this is line   3 in file2.in ...
this is line   6 in file2.in ...
this is line   9 in file2.in ...
 
C:\>type file3.out
this is line   3 in file3.in ...
this is line   6 in file3.in ...
this is line   9 in file3.in ...
 
C:\>
C:\>