Note that unless the first timestamp in the later file exactly matches one of the timestamps in the earlier file, your code will keep counting through the rest of the file.

Use the 3-arg form with lexical filehandles:

open my $fh, '<', 'newfile.txt'
   or die "Can't open newfile.txt: $!\n";
[download]

Dave.

That is exactly correct. The timestamps will eventually match up. Essentially, one of the files may be a subset of the other. Each line has data that may be different for the same times. I need to compare that data. That part of the code is done. So, yes, I can count on the timestamps matching up once I get rid of the first n lines of the earlier file. What is the easiest (prettiest I guess) way to do this? Thanks a lot.

You probably want something like the following (not heavily tested):

use warnings;
use strict;

@ARGV == 2 or die;
my @files = @ARGV;

my $date_qr = qr{^(\d{4}/\d\d/\d\d \d\d:\d\d:\d\d\.\d{3}) };

my @dates;
for my $i (0,1) {
    open my $fh, '<', $files[$i]
        or die "Can't open $files[$i]: $!\n";
    while (<$fh>) {
        next if /Epoch/;
        chomp;
        m/$date_qr/ or die "$files[$i]:$.: invalid line: $_\n";
        $dates[$i] = $1;
        last;
    }
}

print "earliest=@dates\n";

my $i = 0;
if ($dates[0] ne $dates[1]) {
    my $earliest = $dates[0] lt $dates[1] ? 0 : 1;
    open my $fh, '<', $files[$earliest]
        or die "Can't open $files[$earliest]: $!\n";
    while (<$fh>) {
        next if /Epoch/;
        chomp;
        m/$date_qr/ or die "files[$earliest]:$.: invalid line: $_\n";
        last if $1 ge $dates[!$earliest];
        $i++;
    }
}

print "there are $i earlier date entries\n";
[download]

Dave.