This is even documented in perlref:

You're right! (What else?!?)

I'm also ashamed, since I had read that section sometimes... OTOH retrospectively I'm slightly surprised that at least a few experienced perl hackers from clpmisc didn't recognize it at a first glance either...

Because curly brackets (braces) are used for several other things
including BLOCKs, you may occasionally have to disambiguate braces
at the beginning of a statement by putting a "+" or a "return" in
front so that Perl realizes the opening brace isn't starting a
BLOCK.  The economy and mnemonic value of using curlies is deemed
worth this occasional extra hassle.
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Hmmm, it seems that the docs concentrate on the somewhat opposite situation, i.e. that in which a hashref is mistaken for a block.

The leading "+{" and "{;" always serve to disambiguate the expres-
sion to mean either the HASH reference, or the BLOCK.
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I must say that while I find "+{" aesthetically appealing, I can't say the same of "{;". But of course that's just me...