As softworkz and tommyw have said if $0 does not contain a path (ie invoked as ./script.pl) then pwd should be OK -provided you have not changed directories before you read it. However I would suggest that the best solution is to code the location of the config file. I usually end up with something like:

# untested code...
my $config_dir = "/path/to/script/config";
my $config_file = $config_dir . "/" . "config.file";
my %config = read_config($config_file);

sub read_config {
  my $config = shift || die "Config file name not supplied \n";
  open(CONFIG, $config) || die "Could not read from $config: $!\n";
  # code to parse config file goes here
  close(CONFIG);
  return %config;  # or @config or .... 
}
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Then it doesn't matter how your script is invoked, it will always know where to look for its config file.

--
my $chainsaw = 'Perl';