Further to toolic's reply:

if(exists($final->{$x[$i]})){
    print "second\n";
    ($final->{$x[$i]}, $f);
    }
else{
    print "first\n";
    $final->{$x[$i]} = ($final->{$x[$i]}, $f);    
    }
[download]

The statement
    ($final->{$x[$i]}, $f);
does nothing. What do you expect it to do?

The statement
    $final->{$x[$i]} = ($final->{$x[$i]}, $f);
is a bit more complex. It evaluates the list  ($final->{$x[$i]}, $f) in the scalar context imposed by the scalar  $final->{$x[$i]} on the LHS of the assignment. I doubt this statement does what you expect. Please see the Context tutorial, in particular the "Context clash" section.

c:\@Work\Perl>perl -wMstrict -MData::Dumper -le
"my $final = { qw(a 1  b 2  c 3), d => [ 9, 8, 7 ] };
 my @x = qw(a b c);
 print 'data to start';
 print Dumper $final;
 print Dumper \@x;
 ;;
 my $f = 'ZOT';
 ;;
 print 'void-context expression';
 ($final->{$x[1]}, $f);
 print Dumper $final;
 ;;
 print 'assign list in scalar context';
 $final->{$x[1]} = ($final->{$x[1]}, $f);
 print Dumper $final;
"
Useless use of hash element in void context at -e line 1.
Useless use of hash element in void context at -e line 1.
Useless use of private variable in void context at -e line 1.
data to start
$VAR1 = {
          'c' => '3',
          'a' => '1',
          'b' => '2',
          'd' => [
                   9,
                   8,
                   7
                 ]
        };

$VAR1 = [
          'a',
          'b',
          'c'
        ];

void-context expression
$VAR1 = {
          'c' => '3',
          'a' => '1',
          'b' => '2',
          'd' => [
                   9,
                   8,
                   7
                 ]
        };

assign list in scalar context
$VAR1 = {
          'c' => '3',
          'a' => '1',
          'b' => 'ZOT',
          'd' => [
                   9,
                   8,
                   7
                 ]
        };
[download]

Please pay attention to the warning messages generated by warnings. (The "Useless use of..." messages will have useful line numbers associated with them if you run your code from a script rather than from the command line as I have.)

I think there may be other problems with the script, but this should get you going. Perhaps also see Perl Data Structures Cookbook.