Found in perlfaq4

How can I count the number of occurrences of a substring within a stri
+ng?
There are a number of ways, with varying efficiency. If you want a cou
+nt
of a certain single character (X) within a string, you can use the
"tr///" function like so:

        $string = "ThisXlineXhasXsomeXx'sXinXit";
        $count = ($string =~ tr/X//);
        print "There are $count X characters in the string";

This is fine if you are just looking for a single character. However, 
+if
you are trying to count multiple character substrings within a larger
string, "tr///" won't work. What you can do is wrap a while() loop
around a global pattern match. For example, let's count negative
integers:

        $string = "-9 55 48 -2 23 -76 4 14 -44";
        while ($string =~ /-\d+/g) { $count++ }
        print "There are $count negative numbers in the string";

Another version uses a global match in list context, then assigns the
result to a scalar, producing a count of the number of matches.

        $count = () = $string =~ /-\d+/g;
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