ERROR: index row requires 9984 bytes, maximum size is 8191

CREATE OR REPLACE FUNCTION reverse(varchar) RETURNS varchar AS $$  $reversed = reverse $_[0];  return $reversed; 
$$ LANGUAGE plperlu IMMUTABLE;

CREATE INDEX rev_email ON users( (reverse(email) ) varchar_pattern_ops );

I am getting the following error ERROR:  index row requires 9984 bytes, maximum size is 8191

my logs are not showing any thing else , but the error. Can you please help?

Regards