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

my $INPUT = 'blabla.txt';
my $OUTPUT = 'blabla2.txt';
my $FOURTH_LINE = 'z';

open(my $fhi, $INPUT) or die "Cannot open $INPUT : $!";
open(my $fho, '>', $OUTPUT) or die "Cannot open $OUTPUT (writing): $!";

while (defined(my $line = <$fhi>)) {
    print $fho $line;
    print $fho $FOURTH_LINE, "\n" if ($. == 4);
}

close($fhi);
close($fho) or die "Cannot close $OUTPUT : $!";