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 = { z => 300 };

##</code><code>##

300 # from z

##</code><code>##

my $w = values %$hash_w; 
my $x = values %$hash_x; 
my $y = values %$hash_y; 
my $z = values %$hash_z; 

my @all,
push @all, $w,$x,$y,$z;

my $max_val = (sort @all)[0];