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;