use strict;
use warnings;

my $stuff1 = qr/[^A]|[^C-Z]*/;
my $stuff2 = qr/[^A-B]|[^D-Z]*/;
my $stuff3 = qr/[^A-C]|[^E-Z]*/;

my $regex = qr{
A
$stuff1
B
$stuff2
C
$stuff2
D
}x;

while (my $line = <DATA>) {
    chomp($line);
    if ($line =~ $regex) {
        print "Matched: \" $line \"\n";
        } else {
        print "**Did NOT match \"$line\"\n";
    }
}
__DATA__
ABhere is intervening textC D
A B C, lots of text and numbers and equals and slashes DEFG
ABhere IS intervening TEXTC D
A BIG foo is B intervening text CD
A Big Cat interjects itself into text before CD