There's the experimental (?{...}) construct, but I can't think of a way to use that. Why not use length and a slightly different regex? 
while (length($target_letters) != WORD_LENGTH &&
    $target_letters =~ /[^A-Za-z]/) {
    # prompt
}

[download]
Seems clearer to me, as well as *possible*.

In reply to Re: subroutine within regex quantifier? by chromatic
in thread subroutine within regex quantifier? by hagen

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