Both the documentation of 5.8 and 5.10 point out the fact

   @LAST_MATCH_END
   @+      
      ...  
      You can use $#+ to determine how many 
      subgroups were in the last successful match.  
      ...
   @LAST_MATCH_START
   @-      
      ...  
      One can use "$#-" to find the last matched 
      subgroup in the last successful match.  
      Contrast with $#+, the number of subgroups 
      in the regular expression.
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When running the following program with perl v5.8.8 or v5.10.1

pl@nereida:~/Lperltesting$ cat abigail1.pl
#!/usr/bin/perl
local $" = ", ";

"a" =~ /(a)|(b)/;
print "\@- = (@-)\t length of \@- = ".((scalar @-)."\t last - index = 
+$#-\n");
print "\@+ = (@+)\t length of \@+ = ".((scalar @+)."\t last + index = 
+$#+\n")
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produces the same output:

pl@nereida:~/Lperltesting$ perl5.10.1 ./abigail1.pl
@- = (0, 0)      length of @- = 2        last - index = 1
@+ = (1, 1, )    length of @+ = 3        last + index = 2
pl@nereida:~/Lperltesting$ perl5.8.8 ./abigail1.pl
@- = (0, 0)      length of @- = 2        last - index = 1
@+ = (1, 1, )    length of @+ = 3        last + index = 2
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and so the behavior of the perl 5.10 regexp engine is perfectly right. The last matched subgroup is subgroup 1 but there were 2 groups in the last successful match.