If you look at the internals of Exporter::Heavy, you will see the two exports in your example are equivalent to:

# Regular $a, $b export:
*{main::a} = ${Sorts::a};
*{main::b} = ${Sorts::b};

# Glob *a, *b exports:
*{main::a} = *{Sorts::a};
*{main::b} = *{Sorts::b};
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The first one copies only the scalar value slot of the glob, the latter copies all slots. Therefore, I suspect the reason that the glob export works and the other doesn't is that $a and $b don't reside in the normal scalar slots. Update: (Or maybe their "magic" flags don't get copied without the glob. There's a section in perlsub about "localization of globs" that says you can make variables lose their magic by localizing their glob).

The perlvar page indicates $a and $b are special, "Because of this specialness $a and $b don't need to be declared (using local(), use vars, or our()) even when using the strict vars pragma."

Update 2: I believe [id://ysth] has the right answer.