Yes, you can do that; however, you should have a darn good reason for doing so. Why? Because your code will be calling a sub for each and every comparison, which is rather expensive and will slow down your program.

use warnings;
use strict;

my %numbers = qw(one 1 two 2 three 3 four 4 five 5 six 6 seven 7 eight
+ 8);

sort_em(\%numbers);

sub sort_em {
    our %nos =  %{$_[0]};
    print "UP:\n";
    print join "\n", sort { up(\%nos) }  keys %nos; 

    print "\n\nDOWN:\n";
    print join "\n", sort { down(\%nos) } keys %nos; 
}

sub up   { $_[0]->{$a} <=> $_[0]->{$b} }
sub down { $_[0]->{$b} <=> $_[0]->{$a} }
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--perlpelxer