The Perl source code is currently kept in a Git repository, which is browseable through a web interface at https://github.com/Perl/perl5.
The easiest way to copy the Perl repository is to use git:
git clone https://github.com/Perl/perl5.git perl
The master branch, where the development takes places, is named blead. The maintenance branches are named maint-5.XXX depending on the maintenance version.
For more details on working with the Perl source code, please see the perlhack documentation.
If you are interested in released versions of perl, you can download them from the CPAN Perl Source Code page.