Often I will want to assign a variable to be a search and replace of another variable. Logically you might write it like this:

$str = "foofoofoobar";
$new = $str =~ s/foo/FOO/g;

# $new contains 3 because three things were replaced in the string
print "$new\n";

This will not work because you are assigning the number of replacements made to $new. This is not what we wanted. Instead we want the search and replace to return the new string:

$str = "foofoofoobar";
$new = $str =~ s/foo/FOO/gr;

print "$new\n";

Note the /r after the regular expression. Documentation on /r is in Perlop