$var = [
    {
        'id_a' => '1',
        'id_b' => '5',
        'value_x' => '10',
        'value_y' => '5',
    },
    {
        'id_a' => '2',
        'id_b' => '3',
        'value_x' => '20',
        'value_y' => '10',
    },
    {
        'id_a' => '2',
        'id_b' => '3',
        'value_x' => '30',
        'value_y' => '20',
    },
    {
        'id_a' => '3',
        'id_b' => '7',
        'value_x' => '15',
        'value_y' => '15',
    },
];

my $tmp;
my $last_id = '';
for my $hash( @$var ) {
    if ( $hash->{id_a} eq $last_id ) {
        $tmp->[-1]->{value_x} += $hash->{value_x};
        $tmp->[-1]->{value_y} += $hash->{value_y};
    }
    else {
        $last_id = $hash->{id_a};
        push @$tmp, $hash;
    }
}

use Data::Dumper;
print Dumper $tmp;