You are using a C style for loop. The first element of the for loop header is an initialization statement, but $i doesn't actually do anything - it is a variable used in a silly place (void context) because the value isn't used there.

But, you'd be much better to use a Perl loop:

for my $i (0 .. 5006) {
    ...
}
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which makes what is going on much clearer. But for your particular problem you could:

#!/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;
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Note the three parameter open, lexical file handle and a check that the open succeeded.