so I resorted to run the test-scripts manually

I'm not entirely sure of the way you did that ... though I feel I ought to be able to deduce it from the output you provided.
Prior to installing the module, you need to run that test script as perl -Mblib t/polyline.t

If that's what you did, then I'm still puzzled as to the absence of blib in @INC - it *should* be there.

Of course, after you've installed the module, you should be able to run that script as simply perl t/polyline.t

(This may well have little or nothing to do with the problem you're trying to solve, but it's at least a bit of a puzzle in itself.)

Cheers,
Rob