I tried (@\@) but that accepted any old thing (doh!). perlsub does warn you the prototyping capability is limited!

You could always roll your own at run-time, something like (untested):

our %proto = ( '@\@' => sub { return !grep (ref( $_ ) ne 'ARRAY' ), @_
+; },
               # ...
               # proto => sub { enforcement-code }
);

sub foo {
    $proto{ '@\@' }{@_} or die;
    # followed by "real" code for sub foo
}
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