Error: Debian/Ubuntu - Can't locate LWP/UserAgent.pm in @INC

You get an error like this one:
Can't locate LWP/UserAgent.pm in @INC (@INC contains: /home/admispconfig/ispconfig/tools/spamassassin/usr/share/perl/5.10.0 /etc/perl /usr/local/lib/perl/5.10.0 /usr/local/share/perl/5.10.0 /usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.10 /usr/share/perl/5.10 /usr/local/lib/site_perl) at /home/admispconfig/ispconfig/tools/spamassassin/usr/bin/sa-update line 81.
BEGIN failed--compilation aborted at /home/admispconfig/ispconfig/tools/spamassassin/usr/bin/sa-update line 81.


Solution

Install the libwww-perl package:
apt-get install libwww-perl

On Ubuntu you have to prepend the command with sudo if you are not logged in as root:
sudo apt-get install libwww-perl