I tried it with Test::LectroTest::Generator.

#!/usr/bin/perl

use strict;
use warnings;
use Test::LectroTest::Generator 
    qw(:common :combinators);

my $str_gen = String( charset => "a-f", length => [12,] );

for my $string_guidance (1) {
    my $i = $str_gen->generate($string_guidance);
    print "$i ";
}
print "\n";
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Update: Trying to get a work-around for the constraint, I kept coming across warnings that the length cannot exceed the number of chars in the set. If I do length => [6,], then the constraint is automatic, and it works.