use Test::More tests => 4;
use Test::Warn;
use Hash::DefaultValue;
tie my %hash, 'Hash::DefaultValue', 1;
my $key = undef;
warning_is {
no warnings;
is $hash{$key}, 1;
} undef;
warning_like {
use warnings;
is $hash{$key}, 1;
} qr{Use of uninitialized value( \$key)? in hash element};