One way to do it is to just use chr to get the character you want, then use that: 
#!/usr/bin/perl -w

use strict;
use constant DLE=>0x10;
use vars qw($DLE_C);
$DLE_C = chr(DLE);

my $msg = pack ("CCCCC", 0x31, 0x32, 0x10, 0x10, 0x33);
# match $dle's expanded, replacement is not expanded ?
$msg =~ s/$DLE_C$DLE_C/$DLE_C/g;
printf ("%02x " x length($msg) . "\n", unpack ("C*", $msg));

[download]

Another way is to use capturing inside the RE, then just replace with the captured part: 

$msg =~ s/(\x{$dle})\x{$dle}/$1/g;

[download]

As for why it doesn't work the way you expect, Abigail-II seems to have already given a better answer than I could.

In reply to Re: Why is variable interpolation suppressed in \x{$xxx} replacement ? by sgifford
in thread Why is variable interpolation suppressed in \x{$xxx} replacement ? by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.