part of the problem is that $SecOff is somehow undefined, at least when I run it through the debugger.

That's not what it looks like to me. Instead, it would appear to me that @_ somehow is empty, or partly empty, or filled with a few undefined values. At least, the number of warnings and the line number they occur at, would agree with it: pack with empty $_[4] and $_[5] (line 67), addition with empty $_[3] (line 68), $_[5] (line 69), and $_[3] (line 67).

I do not see what could cause that, though. It's an odd error — perhaps even a Perl bug for version 5.8.6. It's the last line in the sub _timegm I currently have under suspicion:

    $sec +  86400 * &_daygm;
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Can anybody confirm @_ is passed unaltered, on 5.8.6, when a sub is called with "&" and without parens?