On my version (5.3) the UserGuide2.pdf, Chapter 9, has several sections dealing with Unicode issues:
“Compatibility with Previous Releases”
“Using Tcl 8.3.1 Unicode Features”
“European Character Set Support”
Seach for the terms like: ‘encoding’, ‘utf-8’, ‘non-ASCII’, ‘fconfigure’
There were various work-arounds. Maybe you don’t need them now with 5.5, or maybe you switched platforms and the system encodings are different. (ISO8859-1 vs CP1252)