Sorry I thought it's obvious, any technique based on separation at delimiters results in an incremented result, so

 $x =split /;/, $string means their are $x-1 ";" in string.

edit

IMHO the only chance to get faster than now is to find an obscure feature in unpack.

update

example:

  DB<103> $_="aXaXa"
 => "aXaXa"

  DB<104> $x=split /X/
 => 3

  DB<105> split /X/
 => ("a", "a", "a")
[download]

ooops! 8-/

  DB<106> $_="XaX"
 => "XaX"

  DB<107> $x=split /X/
 => 2

  DB<108> split /X/
 => ("", "a")
[download]