If you live in a non-unicode world, then tr///was designed for this:
$s = 'This is A mIxEd-Case STRING';;
$s =~ tr[A-Za-z][a-zA-Z];;
print $s;;
tHIS IS a MiXeD-cASE string
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.