1. Install the dependencies

Install PERL using your package manager.

Install the following PERL modules:
* IO::Socket::INET
* Net::Ident

These may be available through your package manager.  If not, then 
install them from CPAN:
# perl -MCPAN -e shell
# install IO::Socket::INET
# install Net::Ident

2. "Install" ident-user-enum.pl

It's just a PERL script, so no installation is necessary, but probably
want to copy it into your path and set appropriate permissions:
# install -o root -m 0755 ident-user-enum.pl /usr/local/bin/