This is a really good effort, but it fails if characters not matching the given character set are included in the string.

Unfortunately, I don't have a solution, either because in my mind, there would have to be a lookbehind on a string of variable length, which (of course) will not work.

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

while(<DATA>) {

        chomp;

        if (! /([smtwhfa])(?=.*?\1)/) {

                print "$_ : OK\n";
        }

        else {
                print "$_ : Not OK\n";
        }
}

__DATA__
swma
smqa
smsa
fhtm
ttma
t2ms

__END__

swma : OK
smqa : OK
smsa : Not OK
fhtm : OK
ttma : Not OK
t2ms : OK
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