I'm using many Perl modules in my script, my concern is while compile the process memory keeps growing drastically. I want to undef some module once after its done.

I'm trying to understand the problem before I suggest a solution. Are you saying while the script's compiling, and before it starts to run, it uses a lot of memory? Or are you saying that when it runs (and compiles first) it uses a lot of memory?

Is this a long-running script, or a daemon? Is it processing large files that are getting read into memory? Is it processing large XML files? Is this a real performance problem? Are you running this script on a machine that's underpowered? Can you get more memory for the machine?

If you can explain what's going on in a little more detail, perhaps we can make better suggestions.