#!/usr/bin/perl

use strict;
use warnings;

my @found;

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

m<
    (
     (?{
         push @found, pos()
       })
     abc   # the string I know which repeats
    )
    \G
    .+?
    \1
 >gx;

print join ', ', @found, "\n";