t/op/catch.t: add comments explaining its purpose

This test file didn't have any comments explaining what it was for:
especially confusing as there's no 'catch' op.

1 files changed, 6 insertions, 0 deletions

diff --git a/t/op/catch.t b/t/op/catch.t
index 2ed6a16d9b..92f3970772 100644
--- a/t/op/catch.t
+++ b/t/op/catch.t
@@ -1,5 +1,11 @@
 #!perl
 
+# Test that exception catching is set up early enough when executing
+# pp_entereval() etc. There used to be a gap where an exception could
+# be raised before perl was ready to catch it.
+#
+# RT #105930: eval 'UNITCHECK{die}' crashes inside FETCH
+
 BEGIN {
     chdir 't' if -d 't';
     require './test.pl';