our $Type ;

BEGIN {
    if    ( $^O =~ /dar/ ) { $Type = 'Mac' ; }
    elsif ( $^O =~ /sol/ ) { $Type = 'Sun' }
    elsif ( $^O =~ /aix/ ) { $Type = 'Aix' }
    else                   { die "unknown os." }
};

package base ;

sub common_routine {}
sub specific_routine { die "must be one of mac, aix, sun" ; }

sub 

package Mac ;
our(@ISA) = qw/base/ ;

sub specific_routine { DoMacStuff()}


package Aix;
our(@ISA) = qw/base/ ;

sub specific_routine { DoAIXStuff()}

package Sun ;
our(@ISA) = qw/base/ ;

sub specific_routine { DoSunStuff()}


package main ;

my($obj) = new $Type ;

$obj->specific_routine ;