my $test_string="1b22a3d3c";

my @ordered;
push(@ordered, $1) while $test_string =~ /([abcd])/cg;

pos($test_string) = 0; # reset m//cg offset

##</code><code>##

my %seen;
my @order = grep { !$seen{$_}++ } $test_string =~ /([abcd])/g;