LanX has a good answer. So, moving on and proselytizing, regular expressions give you enough power that if you feel like you need an internal callback, you are probably using them wrong. For the case of counting 's' in a string, the easiest answer is

$count = $var =~ tr/s/s/;
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as documented in How can I count the number of occurrences of a substring within a string? in perlfaq4. I try to keep regular expressions short and sweet, and if I need really complex logic, I write that in Perl so that it's more than line noise to the casual observer.