Checks if the numbered capturing buffer has matched something. 
 beri beri 
^,               then '(hana)' fails. Fail.

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

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(?(1)...) doesn't make any sense since it's impossible to match the regex without the first capture matching something. Did you mean (?(2)...)? 

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

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

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

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

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Update: Added working examples

In reply to Re: conditional expression testing by ikegami
in thread conditional expression testing by Anonymous Monk

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