in reply to map problem
This is another instance of the problem that a pair of curlies can stand for an anonymous subroutine or a hash reference. You can disambiguate with a semicolon in the block:
my %y = map {; "prefix_$_" => 1 } qw(a b c);
It's more of a design limitation than a bug in perl or in you :-)
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Seekers of Perl Wisdom