my $allusers = [ {
                  'user'      => 'Sarah',
                  'duration'  => 2,
                  'amount'    => 200
               },
               {
                  'user'      => 'William',
                  'duration'  => 1,
                  'amount'    => 100
               },
               {
                  'user'      => 'Michael',
                  'duration'  => 3,
                  'amount'    => 300
               },
               {
                  'user'      => 'Michael',
                  'duration'  => 3,
                  'amount'    => 300
               },
               {
                  'user'      => 'William',
                  'duration'  => 7,
                  'amount'    => 700
               },
               {
                  'user'      => 'Sarah',
                  'duration'  => 5,
                  'amount'    => 500
               } ];

##</code><code>##

$allusers = [ sort { $a->{'user'} cmp $b->{'user'} } @$allusers ];
            
my ($temp, $grouped);
my $ctr = -1; 

for ( @{ $allusers } ) {
   if ( $temp && $temp eq $_->{'user'} ) {
      $grouped->[$ctr]{'amount'}    += $_->{'amount'};
      $grouped->[$ctr]{'duration'}  += $_->{'duration'};
   } else {
      $ctr++;
      $temp = $grouped->[$ctr]{'user'} = $_->{'user'};
      $grouped->[$ctr]{'amount'}       = $_->{'amount'};
      $grouped->[$ctr]{'duration'}     = $_->{'duration'};
      
   }
}

##</code><code>##

my $allkeys;
for  ( @{ $allusers } ) {
    my $user = $_->{'user'};
    if ( !exists $allkeys->{$user} ) {
        my $attributes = { 'amount'    => $_->{'amount'},
                           'duration'  => $_->{'duration'}
                         };
        $allkeys->{$user} = $attributes;
        
    } else {
        my $refAttributes              = $allkeys->{$user};
        $refAttributes->{'amount'}     += $_->{'amount'};
        $refAttributes->{'duration'}   += $_->{'duration'};
    }
}

my $finalArray;
while ((my $key, my $value) = each %{$allkeys}) {
   my $hash = {  'user'       => $key,
                 'amount'     => $value->{'amount'},
                 'duration'   => $value->{'duration'}
               };
   push @{$finalArray}, $hash;
}

##</code><code>##

print Dumper ($finalArray);
$VAR1 = [
          {
            'amount' => 600,
            'user' => 'Michael',
            'duration' => 6
          },
          {
            'amount' => 800,
            'user' => 'William',
            'duration' => 8
          },
          {
            'amount' => 700,
            'user' => 'Sarah',
            'duration' => 7
          }
        ];