My understanding of your question is: "How can I monitor an ever-growing log file until some keyword is found?"

A method (actually 3 methods) is provided by the Perl Cookbook. See the section "Trailing a Growing File" (Section 8.5 in my first edition).

O'Reilly has kindly provided Chapter 8 on the net: http://oreilly.com/catalog/cookbook/chapter/ch08.html. Scroll down to the "Trailing a Growing File" section.