About the perl index function question

[Majian]

Hi,all :

     I have a problem about the perl index function , the problem is the
following :


#!/usr/bin/perl
use strict ;

my $s = "The black cat jumped from the green tree";
print index $s, "e", 3  ####The result is 18


I don't understand  the result clearly ,Could someone help me to explain it?

I also type "perldoc -f index" in terminal  for details, but I haven't
understand yet ~






Thanks~