$a =~    s{\b([A-Z]{4,})\b}{ucfirst(lc($1))}eg;
                         ^^^ slash removed
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You realize of course that there many other (probably better) ways to do this. Particularly, I wonder why you would need the \b, as [A-Z]{4,} is greedy and will take whole words on a match. And then there is of course the \L escape sequence. I would probably have done it like this:

$a =~    s#([A-Z])([A-Z]{3,})#$1\L$2\E#g;
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Liz