ok $exp eq $got or diag explain [$exp, $got];

Yeah, that's not a bad idea.
I don't get much sense from "or diag explain [$exp, $got]" when the variables are Math::MPFR objects:

C:\>perl -MMath::MPFR -MTest::More -e "$got=Math::MPFR->new(2); $exp= 
+Math::MPFR->new(2); ok($got != $exp) or  diag explain [$exp, $got]; d
+one_testing();"
not ok 1
#   Failed test at -e line 1.
# [
#   bless( do{\(my $o = 46472664)}, 'Math::MPFR' ),
#   bless( do{\(my $o = 46474008)}, 'Math::MPFR' )
# ]
1..1
# Looks like you failed 1 test of 1.
[download]

but I could instead use something like or warn "$got == $expected" to provide a more meaningful diagnostic.

There's probably quite a few places in my test scripts where this blow-up could occur, but the blow-up only happens with the overloaded "!=" operator, and then only if the test fails.

Cheers,
Rob