This answer on stackoverflow seems to do what you want (based on the fact that the bitwise & operator has a different output depending on the type of the scalar) with pure perl. I suppose there is a XS module somewhere that lets you know precisely what there is inside the SV.

But as stated in the above post, the fact that a variable has a numerical value does not only depend on its declaration, but also on whether or not it was ever used in numerical context:

perl -MData::Dumper -E '@a = (1, "1", 1, "1", "1o"); say "$a[2]"; say 
+$a[4]+$a[3]; say Dumper \@a'
1
2
$VAR1 = [
          1,
          '1',
          1,
          1,
          '1o'
        ];
[download]

Edit: added "output" in my parenthesis, because why would I write all the words on the first try?