perl plugin / Collectd.pm: Export plugin_write() to Perl.

perl plugin / Collectd.pm: Export plugin_write() to Perl.

plugin_write() accepts, just like the C counterpart, three arguments,
'plugins', 'datasets' and 'valuelists'. In contrast to the C implementation,
all three arguments may either be a single scalar or a reference to an array -
just like the 'plugins' and 'identifiers' arguments of the Perl implementation
of plugin_flush().