$ perl -Mstrict -Mwarnings -le 'use constant X => "a"; my %h = (a => 1); print $h{X}'
Use of uninitialized value in print at -e line 1.

$ perl -Mstrict -Mwarnings -le 'use constant X => "a"; my %h = (a => 1); print $h{+X}'
1