in reply to using Getopt::Long, arguments and no arguments possible?

Works fine for me. (Getopt::Long v2.25) 
                              does @adduser = ()
--adduser                     does @adduser = ('')
--adduser Joe                 does @adduser = ('Joe')
--adduser Joe --adduser Frank does @adduser = ('Joe', 'Frank')
--adduser --adduser Joe       does @adduser = ('', 'Joe')

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It even works find with your join-split (which would be better written as @adduser = map { split /,/ } @adduser;).

In Section Seekers of Perl Wisdom