package Foo;
use strict;

sub new {
  my $class = shift;
  bless {
    _errno  => 0,
    _errmsg => '',
    _foo    => undef
  }, $class;
}

sub set_foo {
  my ( $self, $foo ) = @_;
  if ( $foo <= 0 ) {
    @{$self->{qw/_errno _errmsg/}} = ( 1, 'foo must be positive' );
    return;
  }
  else {
    $self->{_foo} = $foo;
    return 1;
  }
}

sub error  { $_[0]->{_errno}  }
sub errmsg { $_[0]->{_errmsg} }

##</code><code>##

use Foo;
my $thing = Foo->new;
my $success = $thing->set_foo( -2 );

if ( $success ) {
  # do something
}
else {
  # you can also do if ( $thing->error ) {
  die $thing->errmsg;
}