Even in a restrictive environment you can put your own module-directory and install the module there. With simple pure perl modules (no compiler needed) you can simply fetch the module file(s)? and copy it to that directory. Parallel-ForkManager is pure perl, so no problem. You only have to tell perl where to look:

use lib qw(/some/lib/path);
[download]

And even in the most restrictive environment you can simply put the modules code in your scriptfile.

modules with c-code are more complicated. you´re lost when you don´t have access to a compiler, unless you find it prebuilt.

if you have no shell access, these nodes might be helpful: Install Perl Modules Using FTP Without Having Shell Access?, Installing modules without root and shell