It is quite “normal” to see a Perl module is not thread-safe. Considering the factors:

1. To support multi-thread by default might increase load unnecessary, so usually, the threading part would be left to the users’ hands
2. Most of those modules are older than threading support in Perl

Said that, now it would be your responsibility to lock shared variables, in this case, your tail buffer.

Better to have a buffer, which is fully under your control (you are almost on the right track with this), so you can lock/unlock at the right timing (locking is what missing here, do it yourself, don’t expect Tk to do it for you.).

Be careful not try to share Tk component/widget over threads, as far as Perl 5.8 goes, this is not supported (as bless is not thread-safe). Expect this to be straightened in future versions.