Almost right, Yon, not "tcl_" but any global prefixed with "tcl", "error" or the specific global "env" are preserved. Thanks for the tip, I dove into AOLserver sources and in about 30 seconds uncovered the documentation, in nsd/tclinit.c:
"proc " CLEANUP_PROC " {{autoclose 0}} {\n"
" foreach g [info globals] {\n"
" if {![string match tcl* $g]\n"
" && ![string match error* $g]\n"
" && [string compare env $g] != 0} {\n"
" upvar #0 $g gv\n"
" if [info exists gv] {\n"
" unset gv\n"
" }\n"
" }\n"
" }\n"
