Because %PACKAGE:: is a special hashtable containing all global symbols from the given PACKAGE. Via that hash, a hard reference to the desired subroutine can be looked up. The call to UNIVERSAL::can produces the same result. And obviously calling a subroutine by dereferencing a hard reference is allowed under the stricture.
Try the following sometime:
$ perl -MData::Dumper -e'print Dumper(\%main::);'
It's rather interesting to poke around in there.
Basically, a lot of things in Perl (all of OO really, f.ex) are symbolic lookups, so they cannot be evil by nature. What is evil is accidentally using symbolic lookups where you meant a hard dereference. If your code has a bug so that it happens to put a string rather than the hard reference you intended into $var, without the stricture $$var will still work but suddenly becomes a soft reference. If that only happens sporadically, the resulting bugs can be incredibly hard to spot. That's what strict 'refs' catches, and that's why I strongly suggest you reenable it. You're depriving yourself of a very important safety net otherwise.
Conversly, when you really do need a symbolic lookup, you can still achieve it in ways strict
won't complain about. It's just that you explicitly spell out that you do in fact want a symbolic lookup and are fully aware that it's happening.
Makeshifts last the longest.
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