$&      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>
#

Cheers,
KM