my $logfile = '/full/path/to/logfile';
my $badnews;
open (LF, "<$logfile") or die ("open $logfile failed: $!);
flock (LF,1) or die ("flock $logfile failed: $!");    
while (<LF>) {
    $badnews .= $_ if (m/bad news/i); # some work needed
}
close(LF) or die ("close $logfile failed: $!);

if ($badnews) {
    # do something
}