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

##</code><code>##

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

##</code><code>##

my $string = "smasm";
my @array = split(//, $string);
my %hash;
foreach(@array)
{
    $hash{$_} ++;
}
foreach(keys %hash)
{
    if($hash{$_} > 1)
    {
        print "ERROR\n";
        last;   
    }
}