Not quite what the OP wanted. It was required that the word was completely made of allowed characters only. Try this:

while (<DATA>) {
    chomp;
    if (! /([smtwhfa])(?=.*?\1)/) {
       print "$_ : OK\n";
    } else {
       print "$_ : Not OK\n";
    }
}


__DATA__
smsa
smta
stmwhas

BADsmtaEXAMPLE
[download]

And the output

smsa : Not OK
smta : OK
stmwhas : Not OK
 : OK
BADsmtaEXAMPLE : OK
[download]