I'm really too lazy to dwelve into the inheritance tree of DBIx::SQLEngine, but the bottom line is probably that you're inheriting from one class but the $self you get belongs to a different one. As eric256 suggests you could cope with some run-time modification of @ISA, in order to include the real class of $self, e.g.:

sub new {
    my $class = shift;
    my $self  = DBIx::SQLEngine->new(@_);
    no strict 'refs';
    push @{$class . '::ISA'}, ref $self;
    return bless $self, $class;
}
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Before you decide to take this path be sure to read this thread!