So after all I've got still a small problem here:
When I insert the command output into the text window, it writes squares at the end of some lines (probalby instead of some non-printable end-of-line character)

-> do you know how could I format the string to remove such non-printable characters?

UPDATE:
I solved it with a small character replace before I insert it in the ROText widget in the repeat loop:
$buffer =~ s/\x0D\x0A/\n/g;