Large, old modules tend to be full of historical baggage and strange bells and whistles. For example, constant basically does this:

package constant;

sub import
{
    shift;
    die if @_ % 2;              # probably a mistake
    my $caller = caller;
    while (my ($k, $v) = splice @_, 0, 2) {
        *{"$caller\::$k"} = sub () { $v };
    }
}
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but its import function is 100 lines long and places various weird restrictions on constant names.

If you want to learn from CPAN modules, I suggest looking at some of the newer, simpler ones, like ADAMK's ::Tiny modules.