sub str_to_hex {
  my $str = shift;
  my @h;
  for my $s (split(/(....)/, unpack("H*", $str))) {
    if ($s ne '') {
      if (length($s) < 4) {
        $s = ("0" x (4 - int(length($s)))) . $s;
      }
      push @h, "0x" . $s;
    }
  }

  push @h, "0x0000"
    while @h < 4;

  return @h;
}

for (str_to_hex("COM7:")) {
    print $_ . "\n";
}