First you must find the repeating unit. Try this:

use re 'eval';

$_ = q'abczdefzabcghijklzabczaerabrtyuabcethdauthabkudiabc';

if (/([a-z]{3}).+?\1/) {
    $unit = $1;

    m<
    (
     (?{
         push @found, pos()
       })
     $unit
    )
    \G
    .+?
    \1
    >gx;

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