...
my( $p, $i ) = 0;
$p += $i while $i = 1+index $haystack, $needle, $i;
...

##</code><code>##

...
my( $p, $i ) = 0;
do{ 
    $p += $i;
    print "$p : $i, other stuff";
} while $i = 1+index $haystack, $needle, $i;
...

##</code><code>##

my( $p, $i ) = 0;
loop {
    $p += $i;
    last if $i = 1+index $haystack, $needle, $p;
}