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

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

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

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