Many thanks for your kind answers,

It has been most helpful,

I am absolutely convinced it is not a bug:
I just have read the email from Abigail where he points out that the same behavior appears in previous versions of Perl!:

pl@nereida:~$ perl -v

This is perl, v5.8.8 built for x86_64-linux-gnu-thread-multi

Copyright 1987-2006, Larry Wall
[download]

Perl v5.8.8 regexp engine also produces @+ and @- of different sizes:

pl@nereida:~$ perl -wde 0

Loading DB routines from perl5db.pl version 1.28
Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1):   0
DB<1> "a" =~ /(a)|(b)/; print ((scalar @-)."\n"); print ((scalar @+)."
+\n")
2
3
[download]

Many thanks for your help