Anything else that could de-register a signal handler?

Usually, signal handlers are set and unset by assigning to %SIG, so start looking for assignments to %SIG. A text search for "SIG" should be sufficient.

XS code can mess with signals behind your back, and without perl noticing it. If you use XS code, search for signal and sigaction in the XS code.

The POSIX module also implements a wrapper for sigaction, so you may also want to search for sigaction in the perl code.

And the most obvious one: Search for the constant part of the error message ("received, but no signal handler set.") in the code.

A quick CPAN search reveals that the message may actually come from within perl, from the thread handling or the magic handling code. However, perldiag does not document this message. So, it looks to me like you have really messed up perl's internals.

Alexander

Any suggestion how to approach debugging this.

Is it possible that you're calling a module that implements its own local signal handling for something and instead of restoring what was there previously, it just sets them to undef?