$_ = "lothlorien";

m/ (?{ $i = 0 })
   (.  (?{local $i = $i + 1; })  )*  
   lori
   (?{ $result = $i })
/xi;

print "\$result=$result \$i=$i\n";

# this provides the same answer but is IMHO much easier to understand
m/(.*)(?=lori)/ig;
print "pos=", pos;

# this is probably the best solution
print "index=", index($_, 'lori');