I wonder what he was thinking...

The hash-style usage of constant wasn't added until perl 5.7.1, and the defined $name check was around before that. Basically all of 5.7.0's code was kept intact and put in the for loop. The defined $name check was just accidentally (I presume) left inside the for loop.

Here's how it should look.

my $multiple = ref $_[0];

if ( $multiple ) {
  ...
} else {
  unless (defined $_[0]) {
    require Carp;
    Carp::croak("Can't use undef as constant name");
  }
  $constants{+shift} = undef;
}

foreach my $name ( keys %constants ) {
  ...
}
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Not that this matters much, because

use constant undef, 'x';
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will fail anyway. You just get a slightly less clear error message.

lodin