`-i[SUFFIX]'
`--in-place[=SUFFIX]'
     This option specifies that files are to be edited in-place.  GNU
     `sed' does this by creating a temporary file and sending output to
     this file rather than to the standard output.(1).

     This option implies `-s'.

     When the end of the file is reached, the temporary file is renamed
     to the output file's original name. 

excerpt from info sed, q.v.

#!/usr/bin/env bash

# @(#) s1	Demonstrate REAL re-write in place if enough memory, perl.

# Utility functions: print-as-echo, print-line-with-visual-space, debug.
# export PATH="/usr/local/bin:/usr/bin:/bin"
pe() { for _i;do printf "%s" "$_i";done; printf "\n"; }
pl() { pe;pe "-----" ;pe "$*"; }
edges() { local _f _n _l;: ${1?"edges: need file"}; _f=$1;_l=$(wc -l $_f);
  head -${_n:=3} $_f ; pe "--- ( $_l: lines total )" ; tail -$_n $_f ; }
db() { ( printf " db, ";for _i;do printf "%s" "$_i";done;printf "\n" ) >&2 ; }
db() { : ; }
C=$HOME/bin/context && [ -f $C ] && $C sed perl

FILE=data1
cp sacred $FILE

pl " Short perl code, p1:"
cat p1
pe "# --- end of perl code"

pl " Results sed:"
pl " Data file $FILE before (inode: $( stat -c " %i" $FILE )):"
head $FILE

sed -i '1,2d' $FILE

pl " Data file $FILE after  (inode: $( stat -c " %i" $FILE )):"
cat $FILE

# perl
cp sacred $FILE

pl " Results perl:"
pl " Data file $FILE before (inode: $( stat -c " %i" $FILE )):"
cat $FILE

./p1 1,2 $FILE

pl " Data file $FILE after  (inode: $( stat -c " %i" $FILE )):"
cat $FILE

exit 0

% ./s1

Environment: LC_ALL = C, LANG = C
(Versions displayed with local utility "version")
OS, ker|rel, machine: Linux, 2.6.26-2-amd64, x86_64
Distribution        : Debian GNU/Linux 5.0.8 (lenny) 
bash GNU bash 3.2.39
sed GNU sed version 4.1.5
perl 5.10.0

-----
 Short perl code, p1:
#!/usr/bin/env perl

# @(#) p1	Demonstrate feature (minimal).

use strict;
use warnings;

my ( $debug, $f, $file, @all, $first, $last, $i );

$debug = 1;
$debug = 0;

my ($lines_to_delete) = shift || die " Must supply line numbers.\n";
( $first, $last ) = split( /,/, $lines_to_delete );
$first-- ; $last--;
print " delete lines $first,$last\n" if $debug;
$file = shift || die " Must supply file name.\n";

open( $f, "<", $file ) || die " Cannot open file $file for input.\n";

@all = <$f>;
close($f);

open( $f, ">", $file ) || die " Cannot open file $file for output.\n";
for ( $i = 0; $i <= $#all; $i++ ) {
  print " working on line $i\n" if $debug;
  next if ( $i >= $first and $i <= $last );
  print $f $all[$i];
}

exit(0);

# --- end of perl code

-----
 Results sed:

-----
 Data file data1 before (inode:  334057):
Now is the time
for all good men
to come to the aid
of their country.

-----
 Data file data1 after  (inode:  334060):
to come to the aid
of their country.

-----
 Results perl:

-----
 Data file data1 before (inode:  334060):
Now is the time
for all good men
to come to the aid
of their country.

-----
 Data file data1 after  (inode:  334060):
to come to the aid
of their country.