To see what the regexp is doing, you can use the re pragma. use re 'debug' to enable debugging:

perl -Mre=debug -le 'print /(.)[^\1]\1/ ? "match" : "no match"'

Freeing REx: `","'
Compiling REx `(.)[^\1]\1'
size 19 Got 156 bytes for offset annotations.
first at 3
   1: OPEN1(3)
   3:   REG_ANY(4)
   4: CLOSE1(6)
   6: ANYOF[\0\2-\377{unicode_all}](17)
  17: REF1(19)
  19: END(0)
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from this you can see that the \1 is being interpretted as a unicode 1 to check you can:

perl -le '$_ = "B\1B"; print /(.)[^\1]\1/ ? "match" : "no match"'
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and you'll see that that is a "no match"