You spotted a missing ? to make (uit) optional. Should be:

while ( (print "Prompt: "), $line=<STDIN>, $line !~ /^\s*Q(uit)?\s*$/i
+ )
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That way both the single letter q,Q or the full word Quit works. I just made a typing goof and didn't fully test, otherwise I would have seen that a simple "q" didn't work.