Hello regina, Some thoughts:
in /var/log/koha/koha-error_log
Wed Jun 01 05:35:05 2016] [error] [client 192.168.19.199] Can't locate Modern/Perl.pm in @INC (@INC contains: /usr/share/koha/lib /etc/perl /usr/local/lib/perl/5.14.2 /usr/local/share/perl/5.14.2 /usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.14 /usr/share/perl/5.14 /usr/local/lib/site_perl .) at /usr/share/koha/intranet/cgi-bin/mainpage.pl line 22.
This tells you that Koha cannot currently use the Modern Perl Perl module. Does the server have all the Perl dependencies that Koha requires installed? As Pedro suggests, if you used a Debian package it should automatically resolve the dependencies. If you installed using a different method, then you will have to install the dependencies manually. Alternatively, if you have installed all the dependencies, it could be that your Koha instance currently does not find the dependencies. You may have to check that your koha-httpd.conf points to the directory with all the Perl modules you installed. HTH, let me know if you want more info on any of the above :-) Alex