Invoke-Psake anyFile.ps1 anyTask
you'll notice that after this has completed, there is now a $psake variable polluting the global variable space.
It is also enough to import the module in order to get this variable
But this is probably by design and documented. See psake.psm1:
export-modulemember -function ... -variable psake
In order to get rid of the variable, remove the module:
You will notice that some people are accessing that variable after their script has run: #119
As @nightroman says, by design and removing the module removes the variable.