Why make two passes?

D:\Perl\test>perl
use strict;
use warnings;

my %h; @h{'a'..'j'} = 0..9;
print %h, $/;

my ($min,$max) = (keys %h);
($min,$max) = ( $h{$min} < $h{$_} ? $min : $_, $h{$max} > $h{$_} ? $ma
+x : $_ ) for keys %h;

@h{$min,$max} = @h{$max,$min};
print %h, $/;

^Z
a0b1c2d3e4f5g6h7i8j9
a9b1c2d3e4f5g6h7i8j0
[download]

Simply for the laziness of using List::Util. But if you're gonna optimize it, why check both conditions? An element can't be the maximum and the minimum at the same time. You're also pulling the list of all keys twice.

my ($min, $max) = (scalar each %h) x 2;
($min, $max) = (
    $h{$_} < $h{$min} ? ($_, $max) :
    $h{$_} > $h{$max} ? ($min, $_) :
    ($min, $max)
) for keys %h;
[download]

You could even go all out on each to remove the last bit of redundant work, and to reduce memory footprint.

my $min = each %h;
my $max = each %h;

if(defined $max) {
    local $_;
    ($min, $max) = (
        $h{$_} < $h{$min} ? ($_, $max) :
        $h{$_} > $h{$max} ? ($min, $_) :
        ($min, $max)
    ) while defined($_ = each %h);
}
else {
    $max = $min;
}
[download]

That's all cool if your hash is really huge. If it's just a couple pairs, I'll prefer the lazy route any day.

Makeshifts last the longest.