Niggle:

edge case? specs?

To illustrate; note the use of three (In some ways, more precise, I think but am inviting begging for other views, please?) distinct regexen, $stuff1, $stuff2, and $stuff3 and the last line of __DATA__ and the LAST line of output

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
[download]

OUTPUT:

Matched: " ABhere is intervening textC D "
Matched: " A B C, lots of text and numbers and equals and slashes DEFG "
**Did NOT match "ABhere IS intervening TEXTC D"
**Did NOT match "A BIG foo is B intervening text CD"
Matched: " A Big Cat interjects itself into text before CD "

In the last line of __DATA__ an uppercase "C" preceeds another uppercase "C" (penultimate character), yet the regex does not object (i.e., says there's a match).

Update: Someone upvoted this as I was updating it -- to fix mental and typographic glitches; the said updating may have removed what the ++er thought was meritorious. Sorry.