You're double encoding, you use one or the other, not both.

#!/usr/bin/perl --

use strict;
use warnings;

my $base = 'system.sysDescr.0 : DISPLAY STRING- (ascii):  ';
my $one = quotemeta $base;
my $two = "\Q$base\E";
print "$_\n" for $base, $one, $two, "\Q$one\E", "\Q$two\E";
__END__
system.sysDescr.0 : DISPLAY STRING- (ascii):
system\.sysDescr\.0\ \:\ DISPLAY\ STRING\-\ \(ascii\)\:\ \
system\.sysDescr\.0\ \:\ DISPLAY\ STRING\-\ \(ascii\)\:\ \
system\\\.sysDescr\\\.0\\\ \\\:\\\ DISPLAY\\\ STRING\\\-\\\ \\\(ascii\
+\\)\\\:\\\ \\\
system\\\.sysDescr\\\.0\\\ \\\:\\\ DISPLAY\\\ STRING\\\-\\\ \\\(ascii\
+\\)\\\:\\\ \\\
[download]