$a and $b are not always forced into package main (the way punctuation vars are), and are not magic. But they are localized and aliased to the elements being sorted. The localization temporarily removes the imported ones when you import the scalars rather than the globs. When the globs are imported, the localization replaces what's in the scalar slot of the glob, so it affects the exporting package's "copy".

You can see the same effect here:

$ perl -we'$Foo::a = 2; *a = \$Foo::a; for $a (4) { print $a, $Foo::a 
+}'
42
$ perl -we'$Foo::a = 2; *a = \*Foo::a; for $a (4) { print $a, $Foo::a 
+}'
44
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Update: meant to say, the above is consistent whether either or both var names are a or b or not.