sub no_dup_keys {
    die "Odd number of elements" if @_ % 2;

    my %h;
    while ( my ( $key, $new_val ) = splice @_, 0, 2 ) {
        if ( exists $h{$key} ) {
            warn "Duplicate key: $key, old_value: $h{$key},"
               . " new_value: $new_val\n ";
            next; # Remove for rolling overlay.
        }
        $h{$key} = $new_val;
    }

    return %h;
}

my %hash;
BEGIN{
    %hash = no_dup_keys(
        "one" => "1",
        "two" => "2",
        "one" => "3",
    );
}
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