You don't need to match the non-digits when counting digits, the regex engine will happily skip over them. So, you can shorten the line to

my $count = () = /\d/g;
[download]

If you need to count the characters in a tight loop, note that tr is faster than regex match:

my $count = tr/0-9//;
[download]

Hashes are usually used when you're interested in frequencies of the individual characters.

my %freq;
++$freq{$_} for split //;
[download]