use strict;
use warnings;
use List::Util qw(reduce);

my $maxkey = reduce { $hash{$a} > $hash{$b} ? $a : $b } keys %hash;
my $minkey = reduce { $hash{$a} < $hash{$b} ? $a : $b } keys %hash;

@hash{$minkey,$maxkey} = @hash{$maxkey,$minkey};