Basically something that's not .NET code. There are security implications (Unmanaged code can be insecure, as it bypasses most of .NET's security features) and potential stability issues.

Think of it as XS for .NET--while you can do anything you want and interface to some external library or other, there's nothing to stop the code from stomping on the world. It also means you'll need the perl component on any machine executing your perl code in the .NET environment, though I think .NET's packaging scheme takes care of fetching that as need be. (I'm not 100% sure of that, thoug)