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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

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