while (my $line = <DATA>) {
   chomp $line;
   print "$.: $line\n";   ###
   my ($x, $y, $key, $bar) = split /,/, $line;
   print "A: x=$x, y=$y, k=$key, b=$bar.\n"; ###
   if ($key =~ /[^\d]/) {
      print "BOOM: non-numeric key value <$key>\n"; ###
      ... stuff ...
   }
   else {
      print "B: plover=...\n"; ###
      ... other stuff ...
   }
}

##</code><code>##

perl foobar.pl >tmp
grep -E '^BOOM:' tmp | wc -l