If it's only for the first line, use this instead:

use strict;
use warnings;

use Tie::File;

tie my @xfil, 'Tie::File', 'var.txt' or die $!;

$xfil[0] =~ s/the line to be replaced/this is the new line/;

untie @xfil;
[download]

Substantially less cumber. :-)