As others have said, there are a fair few ways to approach this, so I'll just summarise them:

• Trawl through the registry looking (see How to obtain a list of software applications installed on a Win based PC?) at registered programs.
• Use WMI (quicker) to pull out a list of registered programs: Hardware/Software Inventory
• Use an external program like psinfo to pull out a list of registered programs.
• Scan every .exe on the machine and pull out the publisher, program name and version. You can do this with File::Find::Rule and Win32::AdminMisc.
  This is the approach taken with SMS 2003 and many other vendors in commercial inventory/patching systems.
  There's a code example for this up at codeproject.com.

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