in reply to Perl is psychic?!
Yes, Perl is psychic... but not in this case. If you look at perlvar you will see:
$& The string matched by the last successful pattern match (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK). (Mnemonic: like & in some editors.) This variable is read-only and dynamically scoped to the current BLOCK.
So, since .* matched 'foo', $& is set when you use it. If you make your pattern /\d.*/ you will find you will get no output in your same test case.
(root@frodo):/tmp> # cat t.pl 'foo' =~ m/\d.*/; print eval <STDIN>; (root@frodo):/tmp> # perl t.pl $& (root@frodo):/tmp> #