use constant FIRST_NAME => "first_name";
use constant LAST_NAME => "last_name";

my %some_hash = (
   FIRST_NAME() => "john",
   LAST_NAME()  => "doe",
);

my $f_name = $some_hash{FRIST_NAME()}; # caught by perl -c
my $f_name = $some_hash{"frist_name"}; # autovivifies
...