For the time-being, I have if-then'ed a solution:

sub updateprogress {
    my ($self) = @_;
    return unless $self->progressbar;

    my $frac = frac($self->recordi, $self->recordcount);
    $self->progressbar->set_fraction($frac->num);
}
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and then the data processing loop blindly calls this graphic method:

sub manager {
    my ($Operation) = @_;

    my @row = $Operation->getnew;

    my ( @warnings, @fatals );
    $Operation->recordi(0);
    for my $row (@row) {
        $Operation->recordi( $Operation->recordi + 1 );

        $Operation->data($row);
        $Operation->process;

    $Operation->updateprogress;
    }
    ( \@warnings, @fatals );
}
[download]