$s =~ /A[^A-Z]*B[^A-Z]*C[^A-Z]*D/;

##</code><code>##

# Same thing, but built dynamically.
my $re = join '[^A-Z]*', split //, 'ABCD';
$s =~ /$re/;

##</code><code>##

my $temp = $s;
$temp =~ s/[^A-Z]//g;
$temp =~ /ABCD/;