Update: fixed @$^R to @{$^R}. You can use the return value of (?{ ... }) (which is $^R) which is subject to backtracking.

$string =~ m{
  (?{ [ pos ] })      # we start $^R as [ pos ]
  ([a-z]{3})          # find a string, store it in \1

  (?:                 # now do this one or more times:
    .*?                     # match zero or more characters (non-greed
+ily)
    (?{ [ @{$^R}, pos ] })  # append pos() to $^R's arrayref
    \1                      # matching \1 at this location
  )+

  (?{ @loc = @{$^R} })  # then save $^R
}x
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