Take a look at Nagios (formerly NetSaint). You can write plugins in Perl (you may want to run Nagios with the embedded Perl interpreter for optimal performance) , and at least one GPL'ed Oracle plugin is available on the net.
I did a quick search using the keyword "oracle" and got 36 hits.
Writing Perl plug-ins for Big Brother is really quite trivial. All your script has to do is write to a text file in a standard format, and then Big Brother does the rest. The BB documentation even includes instructions on how to do this.
Cheers,
Darren :)
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