I just found an uglier case.

rob@dev01 $ perl -MO=Deparse -e '$x{_$}'
Missing right curly or square bracket at -e line 1, at end of line
syntax error at -e line 1, at EOF
-e had compilation errors.
rob@dev01 $ perl -MO=Deparse -e '$x{_$}}'
$x{$}->_};
-e syntax OK
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Just interpose the _ and $ in $_. In almost 9 years, I've never run into that. I must have typed "{$_}" almost a million times and never mistyped it. *shudders* That's the kind of typo that brings grey hair to men in the teens!

And, then you have $x{y$}, which is transliteration, $x{s$}, which tries to do substitution ... either it quotes or it doesn't!