Here's a contrived example

#!/usr/bin/perl

# Never leave home without these!
use strict;
use warnings;

# Loop through the faux data log, and...
while (<DATA>) {

    # ...find lines that might be interesting...
    if (/--/) { 

        # ... but ignore certain ones, like those
        # that say "logging" or "disconnected".
        unless ( /logging/ || /disconnected/ ) {

            # Print the rest.
            print "NOTE: $_";

        }

    }

}

# Our faux data for this example
__DATA__
081407 start up server
081407 -- fatal error in procedure 1
081407 -- disconnected user
081407 -- logging started
081407 -- not enough space
[download]

This produces:

NOTE: 081407 -- fatal error in procedure 1
NOTE: 081407 -- not enough space
[download]

Hope this helps,
-v