If you don't want to use Test::Exception (which I highly recommend), then the easist Test::More way of doing this would be this:

eval {
    $a->f1( ) 
};
ok($@, '... an exception has been thrown');
like($@, qr/\- parameter1 required/, '... and it is the correct except
+ion');
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I would also recomment when you test the function with parameters, to also wrap it inside an eval, like this:

eval {
    $a->f1("paramter" ) 
};
ok(!$@, '... no exception has been thrown');
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This is the idea behind the lives_ok function in Test::Exception.