Nice. For the record, this can be simplified a little using Perl’s range operator in static context:

use strict;
use warnings;

my @dd = qw/AA JJ CC M 1 1 3 4 D JJ 1 1 D M 3 3 4 D C M 3 3 3 D/;
my @group;

for (@dd)
{
    if ($_ eq 'M' .. $_ eq 'D')    # or:  if (/^M$/ .. /^D$/)
    {
        push @group, $_;
    }
    elsif (@group)
    {
        print join(' ', @group), "\n";
        @group = ();
    }
}

print join(' ', @group), "\n" if @group;
[download]

Output:

17:41 >perl 1752_SoPW.pl
M 1 1 3 4 D
M 3 3 4 D
M 3 3 3 D

17:42 >
[download]

Hope that’s of interest,