To elaborate a little as to what is going on here:

(1)   The g modifier (the /g at the end of the regex ...) refer to global matching as described in this perldoc:   perlre.   These cause the regex to be applied repeatedly to the same string.   (Also see perlretut, an excellent tutorial.)

(2)   The while loops, as written, are using an implied variable ($_ ... see perlvar) which contains the most recent line read from the file.   The regular expression, also, implicitly matches to that variable since no other variable was referred-to in the statement.   By design, Perl allows you to say things with an economy of characters . . .

Although “TMTOWTDI = There’s More Than One Way To Do It,™” I also share the opinion that substitution isn’t necessary ... merely matching, and counting the number of matches found, as demonstrated above by choroba’s post.

HTH ...