I believe this is because the \x{...} is evaluated at compile-time, and the characters between braces aren't interpolated e.g

use strict;
use warnings;

use constant DLE => 0x10;

my $dle = sprintf ("%02X", DLE);

print "before\n";
print "string is: [\x{$dle}]\n";

__output__

Illegal hexadecimal digit '$' ignored at pmsopw_294950.pl line 9.
before
string is: []
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A simpler solution would be just to continue using pack e.g

use strict;
use warnings;

my $msg = pack "CCCCC", 0x31, 0x32, 0x10, 0x10, 0x33;
my $c   = pack "C", 0x10;

$msg =~ s/$c$c/$c/g;

printf ("%02x " x length($msg) . "\n", unpack ("C*", $msg));

__output__

31 32 10 33
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