use strict;
use warnings;
use 5.012;

my $command = '8M5I4D5M';
my $input   = 'M234567MI234IM234M';

while ($command =~ m/(\d+)([MID])/g) {
    my $value = $1;
    my $code  = $2;

    given($code) {
        when ('D') {
            print 'X' x $value;
        }
        when ('I') {
            $input = substr($input, $value);
        }
        when ('M') {
            print substr($input, 0, $value);
            $input = substr($input, $value);
        }
    }
}