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$string =~ /^[smtwhfa]*$/

$string =~ /^[smtwhfa]*(?![smtwhfa])$/
$string =~ /^[s{1}m{1}t{1}w{1}h{1}f{1}a{1}]*$/
$string =~ /^([smtwhfa])(?!\1)*$/

my $string = "smasm";
my @array = split(//, $string);
...
        last;   
    }
}