split is splitting on the un-captured 'B', and since it's not captured...

No, it's the (A) that's not captured. Every group produces a field, as last paragraph in split explains.

If the PATTERN contains capturing groups, then for each separator, an additional field is produced for each substring captured by a group (in the order in which the groups are specified, as per backreferences); if any group does not match, then it captures the undef value instead of a substring 
  DB<3> @foo = split /(A)|(B)/, "1A2B3"


  DB<4> x @foo
0  1
1  'A'
2  undef
3  2
4  undef
5  'B'
6  3

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I'm reading linked node as japhy apologizes for what he did (i.e. making "split" to return empty string), and acknowledges that returning "undef" was correct from the very begining.

In reply to Re: Re-reading history from 2001 by vr
in thread Re-reading history from 2001 / using a capture during split by talexb

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