use strict;
use warnings;
use my_sort_routines;

package myprogram;

my @data = ( 4, 2, 1, 3 );

my @sorted_proto = sort my_sort_routines::prototyped  @data;
my @sorted_qual  = sort my_sort_routines::qualify_pkg @data;

print @sorted_proto, "\n"; #1234
print @sorted_qual,  "\n"; #1234


package my_sort_routines;

sub prototyped ($$)
{
    # Access the arguments directly by indexing @_

    $_[0] <=> $_[1];
}

sub qualify_pkg
{
    # Qualify $a and $b with the name of the calling package
    # Symbolic refs require "no strict refs" here
    # Symbol::qualify could simplify this, but this illustrates
    # how to do it without an additional module

    my $pkg = caller;
    no strict 'refs';
    ${"${pkg}::a"} <=> ${"${pkg}::b"};
}