package Foobar;

my $var;

# If called with no arguments, just returns the value. Otherwise sets and returns.
sub foovar {
  $var = shift if @_;
  return $var;
}

# This version won't allow the value to be set twice
sub foovar_once {
  die "Attempt to foovar more than once\n" if @_ && defined($var);
  $var = shift if @_;
  return $var;
}

1;

##</code><code>##

use Foobar;

if (1) {
  Foobar::foovar(7);
  print "1. The value is now ", Foobar::foovar(), "\n";

  Foobar::foovar(9);
  print "2. The value is now ", Foobar::foovar(), "\n";
}
else {
  Foobar::foovar_once(7);
  print "1. The value is now ", Foobar::foovar_once(), "\n";

  Foobar::foovar_once(9);
  print "2. The value is now ", Foobar::foovar_once(), "\n";
}