Just a bit of (probably excessive) detail...
In order to get the WORD_LENGTH letters as letters,
OR to prompt if I didn't get them, I needed:

while (length($target_letters) != WORD_LENGTH ||
    $target_letters !~ /[^A-Za-z]/) {
    # prompt
}
process the word
[download]

That is, I changed the && to || and reversed the match to !~, now it works like a bought one.

Thanks chipmunk for your suggestions... I'm a bit
of a newbie, and will need to read up on the syntax there.

I know (a little) about array and hash references, including (even littler)
about anonymous ones but I'd been `saving' myself from subroutine refs,
as they make my head hurt a bit at this stage of my vocation 8-)