String replace in Perl - regex

Question

I am trying to deobfuscate code. This code uses a lot of long variable names which are substituted with meaningful names at the time of running the code.
How do I preserve the state while searching and replacing?
For instance, with an obfuscated line like this:
${${"GLOBALS"}["ttxdbvdj"]}=_hash(${$urqboemtmd}.substr(${${"GLOBALS"}["wkcjeuhsnr"]},${${"GLOBALS"}["gjbhisruvsjg"]}-${$rrwbtbxgijs},${${"GLOBALS"}["ibmtmqedn"]}));
There are multiple mappings in mappings.txt which match above obfuscated line like:
$rrwbtbxgijs = hash_length;
$urqboemtmd = out;
At the first run, it will replace $rrwbtbxgijs with hash_length in the obfuscated line above. Now, when it comes across the second mapping during the next iteration of the outer while loop, it will replace $urqboemtmd with out in the obfuscated line.
The problem is:
When it comes across first mapping, it does the substitution. However, when it comes across next mapping in the same line for a different matching string, the previous search/replace result is not there.
It should preserve the previous substitution. How do I do that?
I wrote a Perl script, which would pick one mapping from mapping.txt and search the entire obfuscated code for all the occurrences of this mapping and replace it with the meaningful text.
Here is the code I wrote:
#! /usr/bin/perl
use warnings;
($mapping, $input) = #ARGV;
open MAPPING, '<', $mapping
or die "couldn't read from the file, $mapping with error: $!\n";
while (<MAPPING>) {
chomp;
$line = $_;
($key, $value) = split("=", $line);
open INPUT, '<', $input;
while (<INPUT>) {
chomp;
if (/$key/) {
$_=~s/\Q$key/$value/g;
print $_,"\n";
}
}
close INPUT;
}
close MAPPING;

Answer 1

To match the literal meta characters inside your string, you can use quotemeta or:
s/\Q$key\E/$replace/

Answer 2

Just tell Perl not to interpret the characters in $key:
s/\Q$key/$value/g

Answer 3

Consider using B::Deobfuscate and gradually enter variable names into its configuration file as you figure out what they do.
I'm a little confused about your request to save state. What exactly are you doing/do you intend to do with the output? Here's an (untested) example of doing all the substitutions in one pass, if that helps?
my %map;
while ( my $line = <MAPPING> ) {
chomp $line;
my ($key, $value) = split("=", $line);
$map{$key} = $value;
}
close MAPPING;
my $search = qr/(#{[ join '|', map quotemeta, sort { length $b <=> length $a } keys %map ]})/;
while ( my $line = <INPUT> ) {
$line =~ s/$search/$map{$1}/g;
print OUTPUT $line;
}