There's often overlapping subtitles with XBMC and BBC iPlayer addon. Rather than modify actual addon and losing changes every time it's updated I hacked up script that edits SRT file after it's downloaded from BBC site but before XBMC reads it.
Install Devel::Trace (libdevel-trace-perl in Ubuntu) and launch your program using "perl -d:Trace ./xx.pl" like syntax. This gives you tracing similar to "set -x" on shell scripts.