Probably harmless diagnostic

[plicease]

When building Alien::uPB (which has the appropriate configuration):

twin% perl Build.PL
+ pkg-config --modversion uPB
Created MYMETA.yml and MYMETA.json
Creating new 'Build' script for 'Alien-uPB' version '0.12'
twin% ./Build
Building Alien-uPB
+ pkg-config --modversion uPB
Use of uninitialized value in pattern match (m//) at /home/ollisg/opt/perl/5.30.0-RC2/lib/site_perl/5.30.0/Alien/Base/ModuleBuild/Repository.pm line 22.
Downloading File: https://github.com/mbarbon/upb/archive/a99b98f94f3289c50f653e2ab7d87d2416bbb89b.zip ... Done
Extracting Archive ... Done
Building library ... 
+ cd /tmp/Alien-uPB-0.12/_alien/upb-a99b98f94f3289c50f653e2ab7d87d2416bbb89b
+ make default googlepb CXXFLAGS="-std=c++11" USER_CPPFLAGS="-pthread -fPIC" 
CC upb/def.c
upb/def.c: In function ‘upb_fielddef_descriptortype’:
upb/def.c:1066:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
...

Note the diagnostic:

Use of uninitialized value in pattern match (m//) at /home/ollisg/opt/perl/5.30.0-RC2/lib/site_perl/5.30.0/Alien/Base/ModuleBuild/Repository.pm line 22.

I think this is harmless, and the undef should be checked in ABMB, but need to do some due diligence first. Otherwise it may need to be reported to Alien::uPB.