Quote from [http://www.mozilla.org/projects/bonsai/]:

Bonsai is a really cool idea on how to "see" changes taking place
during a
development cycle. The original implementation was by Terry Weissman, first done in
[TCL], then later ported to [Perl]. It's built to run against [CVS] using
Perl, [MySQL], and
your favorite webserver to display checkin history, log information, diffs, and other
assorted pieces of information in easy to parse [HTML]. 

I haven't found anything about why it got changed to Perl, or if the TCL verson is still around anywhere. ([EMJ])

Roughly the same thing happened to [bugzilla]. I think that Netscape have a lot of Perl hackers and no Tcl hackers is probably the main issue. [[[PT]]]

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[CVSTrac] is an integrated bug-tracking system that does the same thing (and more).