man perl if you're on a system resembling Unix. This will lead you to other important man pages. If you're not on a Unix system, access to the documentation will be different; for example, it might be only in
HTML format. But all proper perl installations have fully-accessible documentation.
You might also try perldoc perl in case your system doesn't have a proper man command, or it's been
misinstalled. If that doesn't work, try looking in /usr/local/lib/perl5/pod
for documentation.
If all else fails, consult the CPAN/doc directory, which contains the complete documentation in various formats, including native pod, troff, html, and plain text. There's also a web page at http://www.perl.com/perl/info/documentation.html that might help.
It's also worth noting that there's a PDF version of the complete documentation for perl available in the CPAN/authors/id/BMIDD directory.
Many good books have been written about Perl--see the section below for more details.