I see a couple of answers like this:

$ echo " x x x x x x x x
x x x x
 x x x x x x x x x" | perl -pe's/x/o/g && ++$c }{ print "Count = $c\n"
+'
Count = 3
[download]

$ echo " x x x x x x x x
x x x x
 x x x x x x x x x" | perl -pe'if ( s/x/o/g ) { ++$c } }{ print "Count
+ = $c\n"'
Count = 3
[download]

That don't accurately count replacements. Use the return value from s///g:

$ echo " x x x x x x x x
x x x x
 x x x x x x x x x" | perl -pe'$c += s/x/o/g }{ print "Count = $c\n"'
Count = 21
[download]