Content text includes 'perl'

Almost embarrassed to submit this issue as it seems a bit wierd, but it's what happens....

When I try to submit some content that includes the text 'perl' I get a 'page not found' error and the content is not submitted. This applies if the text is on it's own or surrounded by other characters, such as in the word 'sperling'. It happens for the both the standard content type 'page' and for an 'archive' one I created myself.

These are the taken from the site status report:
Drupal 5.0-RC-1
Database schema Up to date
File system Writable (public download method)
MySQL database 4.1.21
PHP 4.4.4
Unicode library PHP Mbstring Extension
Web server Apache - Unable to determine your web server type and version. Drupal might not work properly.

What's somebody got against perl....