my @hashes = (
    { key1 => 1, key2 => 4 },
    { key3 => 8, key4 => 4, key5 => 6 },
    { key6 => 3 },
);

sub sum {
    my $sum;
    $sum += $_ for (@_);
    return $sum;
}

@hashes = map { $_->[0] }
    sort { $a->[1] <=> $b->[1] }
    map { [ $_, sum( values %$_) ] } @hashes;

use Data::Dumper;
print Dumper(\@hashes);