Scanning through the perl 5.6 source-code I found some interesting hints, but no direct evidence as to %_ real purpose.

from /ext/Opcode/Safe.pm

By default, the only variables shared with compartments are the "underscore" variables $_ and @_ (and, technically, the less frequently used %_, the _ filehandle and so on).

perlguts has the following mention

    L        (none)              Debugger %_<filename
    l        vtbl_dbline         Debugger %_<filename element