It doesn't pertain to your question but the code:

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

The * is zero or more, not one or more. For one or more use the + sign. (So it would be /^[smtwhfa]+$/) Unless instead of saying

"A user supplied string may contain 1 or more of the characters smtwhfa in any order."

you really meant zero or more. Just a frequent mistake some people make that I thought I would point out.