Then show your test, because it does work:

$ perl -e 'my $string = "foobarfoofoobarfoo"; my $count = $string =~ s
+/foo/foo/g; print $count;'
4
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As you can see in this Perl one-liner, I print successfully the number of occurrences of "foo" in my $string.

Or, perhaps I didn't understand part of your requirement. What do you mean by substring? Possibly you want this:

my $count = $string =~ s/\bfoo\b/foo/g;
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Edit: Yes, from your answer to Kenosis, I see now that I had not understood what you meant when talking about substring, sorry. The last solution I posted above should work in accordance with your needs.