suppress warning for unescaped { in regex

[dch]

Cosmetic but annoying.

Debug: Prefetching cpan resources for package
Debug: Executing '/usr/local/bin/cpanm --list'
Warning: Could not match Unescaped left brace in regex is deprecated, passed through in regex; marked by <-- HERE in m/^\#\s+PREREQ_PM => { <-- HERE \s*(.*?)\s*}/ at /loader/0x1c719c8/App/cpanminus/script.pm line 1199.

It looks like we don't run perl Makefile.PL ourselves -- it results in a much bigger diff, and so I dropped the changes in manually. Let me know if I should do that in preference.