use strict;
use warnings;

my $text = "COM7:";
my @pairs = unpack "(A2)*", uc unpack "H*", $text;

for (my $i = 0 ; $i <= $#pairs ; ++$i) {
    printf "0x%02s%02s\n", reverse $pairs[$i] || 0, $pairs[++$i] || 0;
}
print "0x0000\n";    # :-)

__END__
0x4F43
0x374D
0x003A
0x0000