Error Codes

I am trying to automate the use of HTML Tidy with a perl script, and i
want to log the various errors that are outputted. Basically what i have
found, which you all will know already, is that error codes 0, 1 and 2
mean no warnings, warnings and errors respectively. But i keep getting
an error code 255 as well. Could any of you inform me as to what is
making this happen?
 
Many thanks,
 

John Aled Hughes