It's quite simple really, but if it's your first script, I'm sure it seems very difficult. /me remembers the first scripts he ever wrote and shudders. Something like this might help, assuming that by 'replace the third line' you mean delete it and place something else there:

#!/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;
[download]

---

Update: If by 'replace the third line', you meant execute a regex to do a 'search and replace' type of thing, then you can modify my above code to something more along these lines:

#!/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;
[download]

---

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