We use /opt for that purpose, as suggested by Solaris filesystem(5):
Update To expand on that, we build our own perl and install it under /opt/perl/5.x.x with judicious symlinks so:/opt Root of a subtree for add-on application packages.
We use #!/opt/local/bin/perl as the shebang line in our Perl programs, and add /opt/local/perl/bin to PATH and /opt/local/perl/man to MANPATH. We can then upgrade perl simply by tweaking those three symlinks. Worked fine for the last upgrade./opt/local/bin/perl -> ../../perl/5.8.8/bin/perl /opt/local/perl/bin -> ../../perl/5.8.8/bin /opt/local/perl/man -> ../../perl/5.8.8/man