Well, I've made some mods to your sample code, and taken the trim time on my sample file from 2m26s to 0.4 seconds. Wow!
In the code below, I haven't included the subroutine to calculate the epoch-second date of each line 'cos it's longer htan the rest of the file!
Thank you, Tachyon!
#!/usr/local/perl -w
use strict;
my $file ='current.txt';
my $daystokeep = $ARGV[0];
my $secs_to_keep = $daystokeep * 3600 * 24;
my $now = time();
my $earliest = $now - $secs_to_keep;
my $file_size = -s $file;
my $top = 0;
my $bottom = $file_size;
my $count = 0;
my $max_tries = 100;
open (OLD, "$file") or die $!;
open (NEW, ">new.txt") or die $!;
while (++$count) {
my $middle = int (($top + $bottom) / 2);
seek OLD, $middle, 0;
my $partial = <OLD>;
my $full = <OLD>;
my $next = <OLD>;
if (((linesecs($full)) < $earliest) && ((linesecs($next)) > $e
+arliest)) {
print NEW $next;
print NEW while <OLD>;
exit;
}
if ((linesecs($full)) < $earliest) {
$top = $middle;
}
else {
$bottom = $middle;
}
}
close OLD;
close NEW;