Perhaps another alternative to consider is Test::NoWarnings.

>perl -wMstrict -le
"use Test::More 'no_plan';
 use Test::NoWarnings;
 ;;
 cmp_ok(0, '==', undef, 'is 0 really undefined?');
"
ok 1 - is 0 really undefined?
not ok 2 - no warnings
#   Failed test 'no warnings'
#   at C:/strawberry/5.14/perl/vendor/lib/Test/NoWarnings.pm line 38.
# There were 1 warning(s)
#   (lots and lots of discussion about the warning elided)
#
1..2
# Looks like you failed 1 test of 2.
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