I knew that you could use :: to make sure that a package name wasn't used as a subroutine, but I didn't really understand what it did.

$ perl -MO=Deparse -e '$x = anything_could_go_here::'
$x = 'anything_could_go_here';
-e syntax OK
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Just like single quotes?

TIMTOWTDI I guess...

So we have

q{}
''
=>
::
qw{}
- (as in -bareword)
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And of course

no strict;
$x = bareword;
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not to mention

"" and qq{}
`` and qx{}
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Somebody will tell me that I missed some, I'm sure.