Something like:

use strict;
use warnings;

my $time = time(); my $days = 45;
my ($day, $mon, $mday, $hour, $min, $sec, $zone, $year, $depth, $flag)
+;

while (read(DATA, $_, 45)) {
    if (!$flag) {
        ($day, $mon, $mday, $hour, $min, $sec, $zone, $year, $depth) =
            unpack('A3xA3xA2xA2xA2xA2xA3xA4x14A3', $_);
        # Use module to convert to timestamp $mytime
        if ($mytime > $time - 86400 * $days) {
            print $depth; $flag = 1;
        }
    }
    else {
        ($depth) = unpack('x42A3', $_);
        print $depth;
    }
}

__DATA__
Fri Dec  3 23:07:21 EST 2004 : Depth is : 234Sat Dec  4 00:07:29 EST 2
+004 : Depth is : 123Sat Dec  5 03:07:32 EST 2004 : Depth is : 144
[download]

Problem is, I don't know a good module for creating a timestamp out of the date information. Maybe someone else can insert that part for me?