$_ = $phone_number;
s/[() -]//g;    # Remove seperators, or
#s/[^0-9xX]//g; # Remove non-digits.
s/^1//;

die("Unrecognize phone number format.\n")
  unless /^(\d{3})(\d{3})(\d{4})(?:[xX](\d+))$/;

$area_code = $1;
$exchange  = $2;
$number    = $3;
$extension = $4;

$clean_phone = join('-', $area_code, $exchange, $number);
$clean_phone .= "x$extention" if defined $extention;