The only way of doing so is to accept that the parameters (or at least the first) are passed as an explicite reference. There's no way to prototype a function in perl that takes either two scalars, or two arrays or two hashes as argument, without the arguments getting flatted into a single list.

sub merge ($$) {   
    die "Not the same references.\n" unless ref ($_ [0]) eq ref ($_ [1
+]);
    if ("" eq ref $_ [0]) {
        $_ [0] .= $_ [1];
    }
    elsif ("ARRAY" eq ref $_ [0]) {
        push @{$_ [0]} => @{$_ [1]}
    }
    elsif ("HASH" eq ref $_ [0]) {
        @{$_ [0]} {keys %{$_ [1]}} = values %{$_ [1]};
    }
    else {
        die "Cannot merge type ", ref $_ [0], "\n";
    }
}
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Abigail