package ParentClass;
# [...]
sub init_server {
        my $self = shift;
        # [...]
        $self->{'server'}->setcallback(
                data       => sub { $self->gotdata(@_); },
                connect    => sub { $self->connected(@_); },
                disconnect => sub { $self->disconnected(@_),
        ) || die "Error setting callbacks: $@\n";
        # [...]
}
# [...]