I'd put the various packages in different files. For instance, the file mac.pm:

package mac;
 
use strict;
use warnings;
use Exporter ();
 
our @EXPORT = qw /get_answer/;
our @ISA    = qw /Exporter/; 
 
sub get_answer {
    "mac";
}
 
1;
__END__
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And then the main program:

#!/usr/bin/perl

use strict;
use warnings;

BEGIN {
    if    ( $^O =~ /dar/ ) { require mac; mac -> import; }
    elsif ( $^O =~ /sol/ ) { require sun; sun -> import; }
    elsif ( $^O =~ /aix/ ) { require aix; aix -> import; }
    else                   { die "unknown os." }
};

print "answer is " . get_answer () . "\n";
__END__
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Abigail