First, with a simple substitution '/g'
 $foo: X o y y o y Z;
 $simplecount: 2 (i.e., 'x's changed)
$foo restored: X x y y x y Z

Now, with given/when and =~ /x/g
$foo contains an x
$foo: X x y y x y Z;
 $count: 1;

 $foo contains a y

 Final $count of 'x' in given/when matching 'x': 1

 X x y y x y Z