Here's a hint as to why. Have a look at the output of
perl -Mversion -E "say qv('0.30')->numify(); say qv('0.3000')->numify();" CPANPLUS uses Module::Load::Conditional, which uses version.pm to compare versions and finds the installed Module::Build lacking. The fix is to change line 18 in Module/Build.pm from $VERSION = '0.30'; to $VERSION = '0.3000';