pods for subroutine argument autovivication

pods for subroutine argument autovivication

Here are pod updates for subroutine argument autovivication.
I'll also try to do some additions to the tests.

One small detail noted:

       perl -we 'sub f { print @_ }; f $a[0]'

produces

       Use of uninitialized value at -e line 1.

under Perl 5.003 but is silent under Perl5.003_94.    Do we care?

p5p-msgid: E0w9489-0005YT-00@ursa.cus.cam.ac.uk