my $h = {}; $h->{ "key1" } = 10; $h->{ "key2" } = 20; $h->{ "key3" } = 30; use constant { KEY => "key2", }; my $x = KEY; print $x , " (x)\n"; print KEY, " (KEY)\n"; print $h->{ $x }, " (x)\n"; # ok print $h->{ KEY }, " (KEY)\n"; # error: Use of uninitialized value