How are we defining "really weird"? I think that this case is pretty weird, and happens with or without parens:

use strict;

sub foo(\@;@) { shift; print @_, "\n"; };

my @arr1 = ( [1, 2], 2, 3 );
my @arr2 = (1, 2, 3);

foo( @arr1, @arr2 );
&foo( @arr1, @arr2 );

__END__
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I'll grant you that this example is tremendously cooked, but the issue is real. As you say, the OP might be unlikely to use such prototypes, but the OP is likely to be using the code of others.