# instead of this (which should have "lazy" set as well):
has 'Dbin'      => ( is        => 'rw',
                     isa       => 'Str',
                     builder   => '_build_Dbin',
                     predicate => 'has_Dbin'
                   );

sub _build_Dbin {
    my $self = shift;
    return $self->{Dbase}.'/apps/combinetools/combineprocessing';
}

# consider this instead if you will never set the value of Dbin
# in the constructor. Note the use of $self->Dbase and not $self->{Dbase}.
sub Dbin {
    my $self = shift;
    return $self->Dbase.'/apps/combinetools/combineprocessing';
}