When running Cygwin Perl using libraries that live on UNC paths (e.g. when running prove with the -l option) Perl will croak because it is unable to load the library. An example error message will look something like: t/00.load.............Can't open perl script ""-I//server/sharename/work/mymodule/lib"": No such file or directory The fix is to map the UNC path to a drive letter and use the Cygwin mount point (e.g. '/cygdrive/z/work/mymodule') as the path instead. The command is net use z: '\\server\sharename'