As a follow up to what Masem proposed, this is a code snippet that I used to monitor my Squid cache performance:

#!/usr/bin/perl 

package DateParser;

use Date::Manip;
sub date_mangler {
        $arg = $_[0];
        $times = $_[1];
        $seconds_gm = time();
        $some_day = localtime($seconds_gm);
        $parsed=&ParseDate($some_day);
        $epoch = &UnixDate($parsed,"%s");
        if(not defined $times) { $times = 1; }
        if(not defined $arg) { $arg = 'default'; }

        if(($arg eq '-min') or ($arg eq '-MIN')){
                $epoch -= ($times *60);
        }
        elsif(($arg eq '-hour') or ($arg eq '-HOUR')){
                $epoch -= ($times * 3600);
        }
        elsif(($arg eq '-day') or ($arg eq '-DAY')){
                $epoch -= ($times * 86400);
        }
        else {
                $epoch -= 86400;
        }
        return $epoch;
}
1;
[download]

A little background: Squid uses epoch time (seconds since epoch) as the date format. Calling DateParser::date_mangler (I tried hard to think of a better name, I promise ;) gives you the minumum number of seconds for the date metric (minute, hour) passed in as a parameter.

Thereafter, simply pick all the lines that have a date stamp larger than the one returned from this function..
HTH, and as always, comments on code much appreciated..

Update:File::Tail is exactly what you want for your job, as noted by others. My procedure is used to do analysis on static files that have already been rotated.