Here's a simpler routine that works with Perl 5.7.3. It doesn't work with Perl 5.6.1 because 5.6.1 doesn't have a CONST flag for CVs.

use B;
use constant CONST => 0x0200;

sub is_constant {
    no strict 'refs';
    my $name = shift;
    my $code = *{$name}{CODE};
    my $obj = B::svref_2object($code);
    return 1 if $obj->isa('B::CV') and $obj->CvFLAGS & CONST;
    return 0;
}
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I wonder why B doesn't offer constants for the values in FLAGS and CvFLAGS...

-sam