use strict;
no warnings; # So many that they hide the output.

{
    package Foo;

    sub doit {
        my ($sorter, @keys) = @_;
        return sort $sorter @keys;
    }
}

my @foo = (3, 1, 2);
print '$a: ', join(' ', Foo::doit(sub { $a <=> $b }, @foo)), "\n";
print '@_: ', join(' ', Foo::doit(sub ($$) { $_[0] <=> $_[1] }, @foo)), "\n";

__END__
$a: 3 1 2
@_: 1 2 3