Just off the top of my head, I would do something like this:

package Result;

use overload 'bool' => \&as_bool, '""' => \&stringify;

sub new
{
  if ($_[0] eq __PACKAGE__)
  {
    shift;
  }

  my $self = {
    as_bool => $_[0],
    string  => $_[1],
  };
  bless $self, __PACKAGE__;
  $self;
}

sub as_bool
{
  my $self = shift;
  $self->{as_bool};
}

sub stringify
{
  my $self = shift;
  $self->{string};
}

1;
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Then you can use it like this:

require Result;
sub complex_function {
  # ...
  if ($error_message)
  {
    return Result->new(undef, $error_message);
  }
  return Result->new(1);
}
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With this, the caller wouldn't change much at all:

unless (my $r = complex_function())
{
  print "Error received: $r\n";
}
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Hope that helps. (Completely untested ;->) Also, the way it is now, Result is not a class you can easily derive from ... I leave that as an excersise for other monks ;->