Debian Sid broke Perl

[Noah Misch]

The Debian Sid buildfarm members have dozens of failures over the last day,
because the latest Perl packages caused "perl -V:useshrplib" to report false.
On thorntail, for some reason, "perl5.30-sparc64-linux-gnu -V:useshrplib" does
return true.  I've added PERL=perl5.30-sparc64-linux-gnu to thorntail.