Perl scripts are not "installed." They are simply run through the Perl interpreter. If you want to "install" them, you copy them to your hard drive. That is all.
It's possible that the question was supposed to be "How are Perl modules installed?", in which case the following is my answer:
Some Perl modules can simply be downloaded into the folder where the Perl modules are kept. For others, it's not so simple.
You can use a program called cpan to install stuff from the CPAN repository (i.e. the official place where people publish Perl modules they've written).
Personally I've never got this to work. I think there's something wrong with my proxy settings or something. So instead I have to do this:
(1) Look up the module on the cpan website.
(2) Download it, unzip it, unpack it.
(3) Look for a file called Makefile.PL or Build.PL . You may find both, in which case check the readme file or something to see which one is preferred.
(4) In the command prompt, open the directory where Makefile.PL or Build.PL is stored.
(5) Type "perl Makefile.PL" or "perl Build.PL" . (I believe it doesn't work without the "perl" bit.)
(6) Type "make"
(7) Type "make test"
(8) Type "make install"
Two more remarks about the last few steps. Firstly, you may need to install "make" itself first, I'm not sure. If you're in Windows, you should install nmake instead of make (and change the commands appropriately). And secondly, watch out for error messages. You'll probably get messages saying that some other module is required before you can install this one. In that case, you have to do the whole process for the other module first, then come back to this one. It can take some time to work your way through all the modules you need. But bear in mind not all modules require this elaborate process, so it might be better to try just saving it in your folder first, as suggested in the first line of this answer.