Eh. Really? OK, well, I would advise against this method, but I was determined to get it working using my original unpack version.. so here goes

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";
}
[download]

This outputs
0x434f
0x4d37
0x003a
0x0000