Curlies ambiguity. In places where blocks and expressions are both valid, curlies are normally interpreted as blocks. However, when the code looks like hash construction, the curlies are interpreted as a hash constructor instead.

>perl -ce"map{ qq{@{ $_->[ 0 ] }} => $_ } @a"
syntax error at -e line 1, next char @
-e had compilation errors.

>perl -ce"map{; qq{@{ $_->[ 0 ] }} => $_ } @a"
-e syntax OK
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