Given 4 distinct hashes with only one key-value, how can we find the maximum value, efficiently? Yes, the values of these hashes will be differrent in different running.

my $hash_w = { w => 100 };
my $hash_x = { x => 200 };
my $hash_y = {};   # no value, but in other cases it can have one
my $hash_z = { x => 300 };
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Hmmm, you do not have "4 distinct hashes", but "4 distinct hasherefs", and the fact that they have each "only one key-value" seems to me to be mostly irrelevant. However I cannot help pointing out that the snippet above suggests you may have chosen your data structures poorly.

Whatever,

my @values=map values  %$_, $hash_w, $hash_x, $hash_y,  $hash_z;
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will give you all the values. Now these are only four values, so a whole sort wouldn't probably be too penalizing, but it would still be more than is actually needed. So you could either roll your own max sub or use that from List::Util.