my $num_substitutions = $var =~ s/foo/bar/g;
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Quoting from perldoc perlop:

       s/PATTERN/REPLACEMENT/egimosx
               Searches a string for a pattern, and if found, replaces
+ that
               pattern with the replacement text and returns the numbe
+r of
               substitutions made.  Otherwise it returns false (specif
+ically,
               the empty string).
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-sauoq
"My two cents aren't worth a dime.";