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 beri beri 
^,               then '(hana)' fails. Fail.

 hana    bbs
^(hana)  (bbs),  then '\2' fails. Backtrack;
^(hana)\2,       then '\1' fails. Fail.

'beriberi' =~ m#^(beri)(bbs)?(?(2)\2\1|\1)$#;

 beri        beri
^(beri)(bbs)?\1  $   Match.

'beribbsbbsberi' =~ m#^(beri)(bbs)?(?(2)\2\1|\1)$#;

 beri  bbs   bbsberi
^(beri)(bbs)?\2 \1  $   Match.