#!/usr/bin/perl -w
use strict;
use File::SortedSeek ':all';

my $targetTS=1234567789;  # for instance
open(FILE,"<data.in") or die "Can't open data.in, error $!";

# File::SortedSeek::numeric looks for a numeric key
# It takes an optional sub ref argument to "massage" the
# line from the file to extract the key.
numeric(*FILE,$targetTS,sub {$_[0]=~/^\s*(\d+)/ or return 0; return $1});

# this will return the line that matches $targetTS, or the first line >
$line=<FILE>;

close(FILE);