I have re-written the code to help you get the logic right.

This code is fairly efficient (There is room for improvement, but it would increase complexity), and made more verbose and simplified.
See the discussion in Alternation vs. looping for multiple searches for improvement ideas.
Untested.

Assumes INFILE contents are smaller than INFILE2, AND small enough to fit into memory.

If this is not the case, you need to consider using a database.

#!/usr/bin/perl -w
use strict;
use warnings;

my %subst;
my $data = 'reference.txt'; 
open my $INFILE, '<', $data or die "cannot read $data: $!";

while(defined my $line = <$INFILE>)
{
   chomp($line);
    my ($id, $description) = split /\t/, $line, 2;
    $subst{$id} = $description;
}
close $INFILE;

my $data2 = 'readthisfile.txt'; 
open my $INFILE2, '<', $data2 or die "cannot read $data2: $!";

while(defined my $line2 = <$INFILE2>)
{
   ## chomp($line2); Dont chomp - leave newline in for printing
    my $changes = 0;
    for my $id (keys %subst){
        if ($line2 =~s/$id/$subst{$id}/g){
           # This line was changed
           $changes++;
        }
    }
    print $line2  if $changes; #Prints only changed lines
}
close $INFILE2;
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Oh, and as a general observation, do check the return value of open. Opening a file may fail.

You might consider reading the first file's code - room pairs into a hash (presuming that each numeric code is unique) and open the second file substituting the hash key for the hash value "looked up" in the first file.

"+<" will allow both read and write.

In most cases like this one modifiying an input file is a really bad idea.

Try this code to see if it is what you are looking for.

#!/usr/bin/perl -w

my $data = 'reference.txt'; 
open INFILE, '<', $data;
my %hash = ();
while (<INFILE>) {
    chomp($_);
    my ($id,$description)=split(/\t/);
    #print $id,' << ',$description,"\n";
    $hash{ $id } = $description;
}
#print "\n";

#while ( my ($key, $value) = each(%hash) ) {
#      print "$key => $value\n";
#      }
    
my $data2 = "readthisfile.txt";
open INFILE2, "<", $data2;
while(<INFILE2>) {                    
    chomp($_);
    @parts = split /\t/;
    print "Original \t",++$i,': ',$_,"\n";
    print "Replaced \t",$i,': ';
    foreach $parl(@parts){
        if ($parl =~ m/(.*\{)([0-9]+)(\}.*)/) {
            $parl = $1.$hash{$2}.$3 if exists  $hash{$2};
            }
    print $parl,"\t";
    }
    print "\n";
                    }
[download]

perl "D:\Temp\repalce1.pl"
Process started >>>
Original     1: ABCDE
Replaced     1: ABCDE    
Original     2:                                                       
+                                                                     
+                                                     
Replaced     2: 
Original     3: +P237S    S{301609239}                                
+                                                                     
+                                                                     
+                          
Replaced     3: +P237S    S{kitchen}    
Original     4: +R372W    W{114672573}                                
+                                                                     
+                                                                     
+                          
Replaced     4: +R372W    W{study room}    
Original     5: +K576R    R{20531742}    R{89242146}                  
+                                                                     
+                                                                     
+                                    
Replaced     5: +K576R    R{living room}    R{terrace}    
Original     6: +R1059Q    Q{126723431}    Q{6934272}    
Replaced     6: +R1059Q    Q{126723431}    Q{6934272}    
Original     7: 
Replaced     7: 
Original     8: STRKE
Replaced     8: STRKE    
Original     9: 
Replaced     9: 
Original     10: +T216I    I{301781720}    I{148237434}               
+                                                                     
+                                                                     
+                                       
Replaced     10: +T216I    I{balcony}    I{148237434}    
Original     11: +V275I    I{149632297}    I{47224534}                
+                                                                     
+                                                                     
+                                      
Replaced     11: +V275I    I{149632297}    I{47224534}    
Original     12: +R13H    H{126333615}                                
+                                                                     
+                                                                     
+                          
Replaced     12: +R13H    H{126333615}    
Original     13: +F113L    L{301781720}    L{148237434}               
+                                                                     
+                                                                     
+                                       
Replaced     13: +F113L    L{balcony}    L{148237434}    
Original     14: +G135S    S{147902132}    S{47224534}    S{125864042}
+    S{107921834}                                                     
+                                                                     
+                                                              
Replaced     14: +G135S    S{147902132}    S{47224534}    S{125864042}
+    S{107921834}    
Original     15: +T307A    A{224050516}    A{126333615}               
+                                                                     
+                                                                     
+                                       
Replaced     15: +T307A    A{224050516}    A{126333615}    
Original     16: +L217F    F{149632297}    F{147902132}
Replaced     16: +L217F    F{149632297}    F{147902132}    
<<< Process finished.
================ READY ================
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