The following is untested, as I have no input data to test with.

use strict;
use warnings;


# An input LOG file which holds information about different users.
my $file_name = "QoEWeb_DB_Log.txt";

open(my $sw, '<', $file_name) or die "$file_name: $!";

#Will write the result into an OUTPUT file.
open(my $file, '>', 'output.txt') or die "output.txt: $!";

foreach (
        sort { $a->[8] <=> $b->[8] } map { [ split /:/ ] } <$sw>
) {
        my ($field1, $field2, $key1, $field5) = @$_[2,4,8,6];

        my @file_name1 = ("Carlo_Arnold_2.232_Final.txt", "Sohaib_Ahma
+d_2.225_Fi
nal.txt"); #Input LOG files; Each file holds informaton about Individu
+al user. I
 will be adding about 30 files here.


        foreach my $file (@file_name1) {
                open(my $rw,$file) or die "$file: $!";
                while (<$rw>) {
                        my @ff_array = split(/:/,$_); #Taking required
+ fields fr
om the second set of input files.
                        my $key2 =  $ff_array[0];
                        my $field3 = $ff_array[1];
                        if( $key1 == $key2) # Finding a match between 
+the two in
put files
                        {

                                print  $file "$field1:$field2:$key1:$f
+ield5:$fie
ld3"; #Printing the desired result from both batch of input files.

                        }
                }
                close($rw) or die "ERROR CLOSING FILE: $!";
        }
}
close $file or die "ERROR CLOSING FILE: $!";
close $sw or die "ERROR CLOSING FILE: $!";
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