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$ perl -wE 'use constant XXX => q{a}; my %x = (XXX() => 123); say $x{X
+XX()};'
123
...

$ perl -wE 'use constant XXX => q{a}; my %x = (+XXX => 123); say $x{+X
+XX};'
Use of uninitialized value in say at -e line 1.