None of the above evaluations results in a crash/segfault. I tested with current Moar and JVM and Parrot from Rakudo Star 2015.02.
I added three tests with commit https://github.com/perl6/roast/commit/3957fed267 I'm closing this ticket as resolved.
[perl #115390] Crash (abort or segfault) when using 'but' in a loop
Christian Bartolomaeus via RT Sun, 05 Jul 2015 00:26:58 -0700
