#! perl
use strict;
use warnings;

{
    package FunctionsFactory;

    my $class = ($^O =~ /Win/) ? 'WinFunctions' :
                                 'LinuxFunctions';
    sub new { return $class->new(); }
}

{
    package WinFunctions;

    sub new { return bless {}, shift; }
    sub f   { print "Windows f()\n"; }
    sub g   { print "Windows g()\n"; }
}

{
    package LinuxFunctions;

    sub new { return bless {}, shift; }
    sub f   { print "Linux f()\n"; }
    sub g   { print "Linux g()\n"; }
}

my $funcs = FunctionsFactory->new();

$funcs->f();
$funcs->g();