$txt = "This is a string";
$count = $txt =~ tr/ //; #counts all spaces
$count = () = $txt =~ m/\s/g;   #counts all whitespace characters
$count = () = $txt =~ m/\s+/g;  #counts whitespace occurences (default [split] behavior)

##</code><code>##

$count = tr/ // for ($txt);