Another solution is to change how your function call handles defaults. Depending on how you set it up, they won't get clobbered:

sub functionCall{
    my ($hashref) = @_;

    my %defaults = (
        A => 1,
        D => 4,
    );

    %$hashref = (
        %$hashref,
        %defaults,
    );
}
[download]

You can change this to true defaults if you switch the order to:

    %$hashref = (
        %defaults,
        %$hashref,
    );
[download]