We use /opt for that purpose, as suggested by Solaris filesystem(5):

/opt  Root of a subtree for add-on application packages.
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Update To expand on that, we build our own perl and install it under /opt/perl/5.x.x with judicious symlinks so:

/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
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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.