Why does Perl complain "Useless use of a constant in void context", but only sometimes?

Question

Here is my Perl script and its output:

use strict;
use warnings;

(undef, 1); # no output
(0, 1);     # no output
(1, 1);     # no output
(2, 1);     # "Useless use of a constant in void context at C:\...\void.pl line 7"
(3, 1);     # "Useless use of a constant in void context at C:\...\void.pl line 8"
("", 1);    # "Useless use of a constant in void context at C:\...\void.pl line 9"
("0", 1);   # "Useless use of a constant in void context at C:\...\void.pl line 10"
("1", 1);   # "Useless use of a constant in void context at C:\...\void.pl line 11"

I would expect warnings at every line. What is special about undef, 0 and 1 which causes this not to happen?

Answer 1

Documented in perldoc perldiag complete with rationale:

This warning will not be issued for numerical constants equal to 0 or 1 since they are often used in statements like

1 while sub_with_side_effects();

As for undef, it's a function that has uses even in void context. e.g. undef($x) does something similar to —but different than— $x = undef();. (You normally want the latter.) A warning could be issued for uses of undef without args in void context, but it would require specialized code, and it's simply not needed.