Conclusion 1:

print treats undef as empty string and issues a waring.

Conclusion 2:

print ignores undef, and issues a warning.

Yes, we can distinguish between those. If print was merely ignoring the undef, then wouldn't

$, = ", ";
print "hello", undef, "goodbye";
[download]

print only one comma? In fact, it prints two. So undef is not being ignored. It's considered an active member of the list, and being printed as an empty string, except that we get a warning if warnings are enabled.

OK, it's the scientist in me, coming up with experiments to test the underlying model. {grin}

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.