References to package subroutines are always done symbolically (even with strict 'refs' on), so you only need to change how the package is called.

#!/usr/bin/perl -w
my $pkg;
BEGIN {
    if    ( $^O =~ /dar/ ) { $pkg = 'mac' }
    elsif ( $^O =~ /sol/ ) { $pkg = 'sun' }
    elsif ( $^O =~ /aix/ ) { $pkg = 'aix' }
    else                   { die "unknown os." }
};

print "answer is " . $pkg->get_answer() . "\n";
exit 0;

package mac;
sub get_answer { return "Apple" }

package sun;
sub get_answer { return "Sun" }

package aix;
sub get_answer { return "IBM" }

__END__
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Which makes it a lot easier to add new class methods in the future.