Others have suggested to unbuffer your writes, but unbuffered writes often make that your program is accessing the disk far more often, slowing down the entire box.

Instead of using '-s', you can find the length of the file also by seeking to the end, then using tell. No unbuffering needed. Here's an example program:

#!/usr/bin/perl

use strict;
use warnings;

use Fcntl qw /:seek/;

my $file = "/tmp/flup";

open my $fh => "> /tmp/flup" or die;

print $fh "Some line of text\n";
my $l0 = -s $file;

# General concept, assuming the filepointer can be anywhere.
# If you know the file pointer is at the end, a simple tell()
# will do, and no seeking at all is necessary.
my $old = tell $fh or die "tell: $!";
seek $fh, 0, SEEK_END or die "seek: $!";
my $l1 = tell $fh;
die "Nothing in file!\n" unless $l1;
seek $fh, $old, SEEK_SET or die "seek: $!";

print "-s $file: $l0. seek: $l1\n";
__END__
-s /tmp/flup: 0. seek: 18
[download]

Abigail