#!/usr/bin/perl -w
use strict;
use Data::Dumper;

my %hash = (
    Matt => 24, Dana => 19, Eric => 28, Sara => 20, John => 17, Mike => 23,
);

my @sorted = sort{ $hash{$a} <=> $hash{$b} } keys %hash;

@hash{ @sorted[0,-1] } = @hash{ @sorted[-1,0] };

print Dumper \%hash;