Well, i hope it's not bad form to answer your own question...

Two (so far) very good replies, but in the meantime i found something even easier|better, i think.

This node is the one i was looking for, and it's all about Tie::File, which essentially turns any flat-file into an array manipulable with Perl in the usual fashion. Absolutely perfect for what i needed.

Here's a stripped-down version of the code i ended up generating:

tie @log, 'Tie::File', $logfile or displayError("could not open logfil
+e");

while ($some_condition) {
    #do some other stuff, and
    unshift @log, $info_to_be_logged;
}

# find out how long the logfile is, and truncate it
$length = @log;
$truncate = $length - 150;
if ($truncate > 0) {
   $#log -= $truncate;
}
    
untie @log;
[download]

By using unshift, each new entry is made at the beginning of the array/file. By truncating the array/file at a fixed number of records before closing, the file essentially becomes FIFO. Of course this isn't perfect, the logfile can swell up a lot while in use, before it gets truncated and there isn't any locking (unless it's built into Tie::File, it seems to be one mightily sophisticated module), but i'm not too concerned.

For the curious, here's a copy of the actual sub that it's used in.... yes, this is my 'production' code, and i know it's pretty horrible ;-)

sub delete_threads_from_file
{
    $logfile = "$htdocs${bbs_location}/$removed_log";
        
    tie @log, 'Tie::File', $logfile or displayError("could not open lo
+gfile");
    

    local($file, $delete_hash) = @_;

    local(*FILE);
    local($new_content);
    if ((open(FILE, "$file"))) {
        while($line = <FILE>) {
            if ($line =~ /<!-- mid="(.*?)" -->/) {
                local($id) = ($line =~ /<!-- mid="(.*?)" -->/)[0];

                $should_delete = 0;
                foreach $delete_id (keys(%{$delete_hash})) {
                    # look for exact id matches
                    if ($id eq $delete_id) {
                        $should_delete = 1;
                        # add this id to the removed.log
                        unshift @log, $delete_id;
                    }
                    # if the delete_id(message) is a topic message,
                    # remove all its responses
                    local($d_thread_id,$d_message_id) = split(/_/, $de
+lete_id);
                    local($thread_id,$message_id) = split(/_/, $id);
                    if (($d_thread_id == $d_message_id)&&($thread_id =
+= $d_thread_id)) {
                        $should_delete = 1;
                        # if this is a response to a removed topic, 
                        # add it to the removed.log
                        if ($id ne $delete_id) {
                            $should_delete = 1;
                            unshift @log, $id;
                        }
                    }
                }

                if (! $should_delete) {
                    # this id is NOT in our list of ids to delete, so 
+keep it
                    $new_content .= $line;
                }
            }
            else {
                $new_content .= $line;
            }
        }
        close(FILE);
        
    # find out how long the logfile is, and truncate it
    $length = @log;
    $truncate = $length - 150;
    if ($truncate > 0) {
        $#log -= $truncate;
    }
    
    untie @log;    
    }


    ###
    ### modify the file
    ###
    if ((open(FILE, "> $file"))) {
        print FILE $new_content;
        close(FILE);
    }
}
[download]