apt-file search "LWP::UserAgent"
This will return the following list on Debian Lenny:
Now you know that in order to satisfy a dependency on "LWP::UserAgent" you would need to install the 'libwww-perl' module from the repo.
If the module is not available via the repo you should next consider installing the package into a custom perl lib directory specifically for the software (and/or user account) in question. To do this read the documentation for the local::lib module in CPAN.
Ultimately your goal should be to not install non-packaged software to /usr/local and by doing so increase the maintainability of your systems.