$#- gives the number of the last capture in the last regex that was successful. With simple alternation as in this case you can just make each alternative a caputure. 
if ( /(foo)|(bar)|(baz)/ ) {
  print "$#-\n";
}

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For details, see the description of @- in perlvar.

In reply to Re: Finding out which of a list of patterns matched by nobull
in thread Finding out which of a list of patterns matched by lemnisca

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