Instead of checking for

defined %{ 'some::package::' }
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... which has been deprecated for a long time and subsequently been removed, why not check whether there are any elements within the package?

perl -wle "package foo; sub unknown {}; package main; print for keys %
+{ 'foo::' }"
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In other words, keys %{ 'some::package' } should tell you whether a package exists, or rather, whether it contains any things of note (to Perl).

Update: perl56delta tells us:

defined(%hash) is deprecated

(D) defined() is not usually useful on hashes because it checks for an undefined scalar value. If you want to see if the hash is empty, just use if (%hash) { # not empty } for example.