I would use configuration file instead. Path to configuration file can be:

• some global known path like /etc
• home dir subdirectory, example: ~/.myapp
• Path relative to main application .pl file location - see Perl FindBin core module

UPD

But if you want to modify .pm at build time - at least, please, write another .pm file from scratch. (like MyConfig.pm) it should contain only variables, nothing except that.