/usr/lib/tcltk/udp1.0.8/pkgIndex.tcl is in libudp-tcl 1.0.8-6.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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package ifneeded udp 1.0.8 \ [list load [file join $dir libudp1.0.8.so]]