How to monitor the progress of a stored procedure?

What techniques can one use to monitor the progress of a stored procedure?  

I have a stored procedure that has been running for a very long time.  Before doing the current "real" run, I tested the procedure on a toy case and confirmed that it produced the right result.  So I know that when it finishes it will be OK, but I can't tell whether it will finish in an hour or in a century...  I have no idea of how far along it is.  (In case it matters, the procedure is written in PL/perl.  I tried to use Perl's print statement, unbuffered, to print out a message periodically from within the procedure, but I see no output.)

Thanks in advance,

Kynn