if( my $foo = bar() ) {
   # $foo isa Foo
} else {
   # $foo isa Error
}

#!/usr/bin/perl

use strict;
use warnings;

my $e1 = Error->new;
if($e1) { print "Error object: $e1"; }
else { $e1->message; }

unless(my $foo = bar())
{
        $foo->message;
}

sub bar {
        return Error->new;
}

package Error;
use overload '""' => sub { '' };

sub new {
        return bless {}, shift;
}

sub message {
        print "Error message!\n";
}

1;