If you are trying to install Spamassassin but get that error, then it means you require the HTML/Parser.pm perl module.
RedHat:
perl -MCPAN -eshell install HTML::Parser
If that gives you problems, run “unset LANG” then try again.
FreeBSD:
cd /usr/ports/www/p5-HTML-Parser make install clean
or to manually compile from source for either OS:
wget http://search.cpan.org/CPAN/authors/id/G/GA/GAAS/HTML-Parser-3.35.tar.gz tar xvzf HTML-Parser-3.35.tar.gz cd HTML-Parser-3.35 perl Makefile.PL make make install