...

my $mod = 'Some::Package::Name';
my $m   = 'method_tested_in_this_test_file';

# testA
{
    my $o = $mod->new;
    is $o->$m($arg), 1, "return val of $m with arg $arg ok";
    ...
}

# testB
{
    my $o = $mod->new;
    is eval {$o->$m('blah'); 1}, undef, "$m croaks if sent in 'blah'";
    like $@, qr/idiot!/, "...and error message is sane";
    ...
}

...