Only a couple of style notes. while($a ne "") is better written (in this case) as while($a).

The print join(" ", @a) is almost always (as least where you're being obscure) better written as print "@a".

Improvements?

This was a pretty good first attempt. I'll only make two suggestions.

The code in the while loop b(substr($a,0,2);$a=substr($a,2); can be tightened up a bit with b $a=~/(.{2})/;$a=$'. (Hope that pasted right). The same formula can be used in b(). Since b() isn't returning a value at all, adding a shift to the very end of it, and then in the while loop saying:

   while($a) {
        $a=b $a=~/(.{2})(.*)/g;
   }
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Tightens things up nicely.

This further shortens up to:

1 while($a=b $a=~/(.{2})(.*)/g);
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