Why all the confusion to avoid use Debug;? It's a bad idea.

If I'd want to do it, I'd just create the sub in the main namespace and use :: in front of the sub name to look into that namespace.

package Debug;
sub ::dp { print "Moo!\n"; }

package test;
::dp;
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$ perl a.pl
Moo!
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If you're truly insane, you could take advantage of the fact that certain symbol default to the main namespace instead of the current namespace.

package main;
sub _ { print "Foo!\n" }
*% = sub { print "Bar!\n" };

package test;
_();
&%();
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$ perl a.pl
Foo!
Bar!
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Basically, any of the special symbol that's allowed as one-character var names (see perlvar) is can be used that way.

The catch is that except for _, they're not normally allowed as identifiers. You'll need to use the &name(...) syntax to call them, and you'll need to assign to the glob to define them.

But that's a bad idea.