(though i guess DOUBLE_BYTE could be a constant, in which case the key is really the constant's value.)

Actually, it's not:

use constant a => 'b';
my %foo;
$foo{a} = "x";
print keys %foo;
__END__
a
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By that logic, it should print 'b' instead. Remember, "constants" in Perl are really just subroutines, not C-style macros. The rule that a bareword as a hash key is always treated as a string applies whether or not there's a subroutine or constant with that name defined.