#!/usr/bin/perl -Tw
use strict;

my $file = 'data.txt';
my $fh;

open $fh, '<', $file
 or die "Could not open $file for read: $!";
chomp( my @lines = <$fh> );
close $fh;

@lines = ('Replace line 3 with this', @lines[3..$#lines]);

open $fh, '>', $file
 or die "Could not open $file for write: $!";
print $fh join("\n", @lines);
close $fh;

##</code><code>##

#!/usr/bin/perl -Tw
use strict;

my $file = 'data.txt';
my $fh;

open $fh, '<', $file
 or die "Could not open $file for read: $!";
chomp( my @lines = <$fh> );
close $fh;

$lines[2] =~ s/foo/bar/g;
@lines = (@lines[2..$#lines]);

open $fh, '>', $file
 or die "Could not open $file for write: $!";
print $fh join("\n", @lines);
close $fh;