Reading the doc, I think you need a ":std" pragma as well, when you are printing to STDOUT. From "open" pragma doc.
The :std subpragma on its own has no effect, but if combined with the :utf8 or :encoding subpragmas, it converts the standard filehandles (STDIN, STDOUT, STDERR) to comply with encoding selected for input/output handles. For example, if both input and out are chosen to be :utf8, a :std will mean that STDIN, STDOUT, and STDERR are also in :utf8. On the other hand, if only output is chosen to be in :encoding(koi8r), a :std will cause only the STDOUT and STDERR to be in koi8r. The :locale subpragma implicitly turns on :std