You could use a hash rather than an array to more easily get your unique left-column items.

use strict;
use warnings;

use Data::Dumper;

my %uniques = ();
while ( <DATA> )
{
    chomp;
    my ($key, $value) = split m{\|};
    $uniques{$key} += $value;
}

print Data::Dumper->Dump([\%uniques], [q{*uniques}]);

__DATA__
10|10
20|20
30|30
10|100
15|15
50|50
15|150
[download]

This produces

%uniques = (
             '50' => 50,
             '30' => 30,
             '10' => 110,
             '15' => 165,
             '20' => 20
           );
[download]

I hope this is helpful.

Cheers,

JohnGG