I am the human manual.

$ perldoc -t perlrun | less +/PERL5LIB
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PERL5LIB
A list of directories in which to look for Perl library files before looking in the standard library and the cur- rent directory. Any architecture-specific directories under the specified locations are automatically included if they exist. If PERL5LIB is not defined, PERLLIB is used. Directories are separated (like in PATH) by a colon on unixish platforms and by a semicolon on Windows (the proper path separator being given by the command "perl -V:path_sep"). When running taint checks (either because the program was running setuid or setgid, or the -T switch was used), nei- ther variable is used. The program should instead say: use lib "/my/directory";