That's an odd one. I've reduced it down to the following:

use strict;

my $ARG1 = shift();
my $ARG2 = shift();

open(F,$0);
my $N;

map {
  print "TRACE $_";
  my @a = (1);
  $N = @a[1..0]; 
  #1;
} <F>;
close(F);

print "\nN = /$N/\n";

#WATCH FOR THIS LINE!
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It appears as though $N is getting set to the return value from the map operation. Notice that I added a commented out line in the map block. If you uncomment this, the value of $N will always be 1.

I don't know what the answer is, but I thought this was worth mentioning. Also, changing this to a while doesn't have this problem, so I think it definitely has something to do with the return value from the map function.

HIH