use strict;
use warnings;
use lib 'lib';

my $arg = shift // ''; 

if ($arg eq "load") {
  print "Now loading optional library.\n";
  require Foo;
  my $o = Foo->new;
  # Do stuff with $o here..
}
else {
  print "I skipped loading the optional library.\n";
  print "Try passing the 'load' option to load it.\n";
}

##</code><code>##

package Foo;
use strict;
use warnings;

BEGIN {
  printf "BEGIN called from package %s\n", __PACKAGE__;
}

print "Class level logic.\n";

sub new {
  my $class = shift;
  $class = ref($class) || $class; # subclass boilerplate.
  print "constructor invoked for $class\n";
  return bless {}, $class;
}

1;