Programming Perl, 4th Edition

Restrictions on constant

List constants are not currently inlined the way scalar constants are. And it is not possible to have a subroutine or keyword with the same name as a constant. This is probably a Good Thing.

You cannot declare more than one named constant at a time as a list:

use constant FOO => 4, BAR => 5;    # WRONG

That defines a constant named FOO whose return list is (4, "BAR", 5). You need this instead:

use constant FOO => 4
use constant BAR => 5;

or even:

use constant {
    FOO => 4,
    BAR => 5,
};

You can get yourself into trouble if you use a constant in a context that automatically quotes bare names. (This is true for any subroutine call, not just constants.) For example, you can’t say $hash{CONSTANT} because CONSTANT will be interpreted as a string. Use $hash{CONSTANT()} or $hash{+CONSTANT} to prevent the quoting mechanism from kicking in. Similarly, since the => operator quotes its left operand if that operand is a bare name, you must say CONSTANT() => "value" instead of CONSTANT=> "value" .