Hmmm - Moritz & Anonymous Monk got me thinking, and I changed it to:

  BEGIN {
      use CGI::Carp qw(carpout); 
      open(ERROR_LOG, ">>my_error_log") 
                      or die("my_error_log: $!\n");
      carpout(\*ERROR_LOG); 
  }
[download]

And low & behold, there's my error output - mostly uninitialized values used in comparisons and concatenations. I've been pasting together old subroutines and some variables used in the subroutines but not in my application are not getting initialized for a few records.

I'll fix all that and see if it then runs without this.

Thanks.

Update: yep, that did it.