for my $i (0 .. 5006) {
    ...
}

##</code><code>##

#!/bin/perl
use strict;
use warnings;

my $thing = join '', map{qw(a t c g)[rand 4]} 1 .. 5007;
my $fName = "./test";

open my $outFile, '>', $fName  or die "Can't create '$fName': $!\n";
print $outFile $thing;
close $outFile;