It is a pretty crappy example but what it does is include some code in the match regex which gets executed. Basically we try for a match with (pseudocode)m/.{0,length $_}lori/ and use $i to remember how many chars we needed to eat up to find "lori". There are far easier ways to do this like using pos() and a positive lookahead assertion if you want a regex solution or just plain old index():

$_ = "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');
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You can't localise and use $i++ in the same statement.

Update

Fixed technical inexactitude thanks jryan

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print