It must have been a bug which was subsequently fixed. I can reproduce your results with: 
This is perl, v5.10.0 built for x86_64-linux

[download]
but not with: 
This is perl 5, version 12, subversion 0 (v5.12.0) built for x86_64-li
+nux

[download]

In reply to Re^3: Inconsistent captures in %+ and %- by toolic
in thread Inconsistent captures in %+ and %- by eff_i_g

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