> I feel like I am missing something very basic here, but I just can't see it. TIA for any suggestions.

You are getting bitten by what is called "slasheritis" in the emacs-lisp world.

You have to escape backslashes from two interpretations which turns many elisp regexes into an unreadable mess filled with quadruple backslashes \\\\

This works:

#!/usr/bin/perl
use strict;
use warnings;
use Test::More;

my ($text, $search);


$text = "\\x1\\x2\\x3\\x4";
$text =~ s/x2\\.+\\x4/new/;
my $exp = $text;


$text   = "\\x1\\x2\\x3\\x4";
$search = "x2\\\\.+\\\\x4";
$text =~ s/$search/new/;
my $got = $text;


is( $got, $exp, "slasheritis");

done_testing;
[download]

ok 1 - slasheritis
1..1
[download]

there are more elegant solutions in Perl°, but I wanted to clarify the problem.

HTH! :)

°) alas not in GnuEmacs. XEmacs had "raw strings", but became abandon-ware.