use strict;
use warnings;

my $stuff = qr/[^A-Z]*/;
my $regex = qr{
  A
  $stuff
  B
  $stuff
  C
  $stuff
  D
}x;

while (my $line = <DATA>) {
    if ($line =~ $regex) {
        print "Matched: $line";
    }
}
__DATA__
ABhere is intervening textC D
A B C, lots of text and numbers and equals and slashes DEFG
ABhere IS intervening TEXTC D