To avoid having to keep on testing against a weird initial value or undefined (-infinity not being allowed), this method is inelegant but seems efficient:

my $max = max( values( %hash_w ) ); # first one separately
for my $href ( \%hash_x, \%hash_y, \%hash_z ) {
    my $try = max( values( %$href ) );
    ( $try > $max ) and $max = $try;
}
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