use strict;
use warnings;
use diagnostics;

our @found;

#              0         1         2         3         4         5
#              012345678901234567890123456789012345678901234567890
my $string = q'abczdefzabcghijklzabczaerabrtyuabcethdauthabkudiabc';
#              abc     abc       abc          abc              abc
#              0       8         18           31               48

if( 
  $string =~ m/
                (.{3})
                (?:
                  .*?
                  (\1)
                  (?{ push @found, pos() - length($^N); })
                )+
             /x
) {
  print "$1: @found\n";
}