正则匹配的问题

CODE:

#!/usr/bin/perl
use strict;
use warnings;
my $digit = "12345678";
my @aa = $digit =~ /(?{print "Staring match at $`|$'\n"})(?=(\d\d\d))/g

QUOTE:

Staring match at |12345678
Staring match at |12345678
Staring match at 1|2345678
Staring match at 1|2345678
Staring match at 12|345678
Staring match at 12|345678
Staring match at 123|45678
Staring match at 123|45678
Staring match at 1234|5678
Staring match at 1234|5678
Staring match at 12345|678
Staring match at 12345|678
Staring match at 123456|78
Staring match at 1234567|8
Staring match at 12345678

CODE:

?=
?!
?<=
?<!