I would echo the comments you already have about the debugger flag. Another thing I have found with XS is that I have had trouble getting perl to run in the debugger (admittedly this is mostly when I am messing with interrupts and with modules linked to dynamic libraries). A trick that I have found usefull is to temporarily insert an infinite loop at the start of my C code:

int temp_val;
temp_val = 100;

/* An infinite loop that usually won't be optimised away */
while(temp_val > 0)
  {
    temp_val--;
    if(temp_val < 50)
        temp_val = 100;
  }
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Then start up the perl script, it will go into the infinite loop. Use

to get its process number. Start gdb with

And get out of the infinite loop with

Now you have a gdb session with all the correct libraries loaded stopped at the start of your routine.