# caller:
StartCapture( { twigs => {filter => $filterTwig, capture => $captureTwig},
                hashes => {filter => \%filterHash, capture => \%captureHash},
                files => {filter => 'filter.txt', capture => 'capture.txt'},
                results => 'c:/temp.txt' } );

# ...

sub StartCapture
{
    my $hashref = shift;

    $$hashref{results} ||= 'c:/SomeDefault.txt';

#   ...
}