This is fun. Math-BigInt-GMP fails tests on Perl 5.26 The version of Math-BigInt-GMP that doesn't fail tests is 1.6001 or greater. Math-BigInt-GMP-1.6001 and greater require Math-BigInt-1.999.801 or greater, which implies Perl 5.26 Subsequently, Math-BigInt-GMP-1.6002 cannot be stabilized without stabilizing perl 5.26, and vice versa.
Created attachment 501178 [details] Math-BigInt-GMP-1.380.0:20171031-001520.log
Created attachment 501180 [details] Math-BigInt-GMP-1.490.0:20171031-001606.log
This is handled in bug 673542.