Did you try printing the value in $date on line 29? The answer is that some of the values are not what you think, because the following code works fine:

#! /usr/bin/perl
use strict;
use warnings;
use Time::Local;

my $iso_time = '2014-03-18T15:41:14Z';
my $expected_epoch = 1 * 60 * 60 + 1 * 60 + 1;
my ($date, $time) = split /T/ => $iso_time;
my ($year, $mon, $mday) = split /-/ => $date;
my $currenttime = time; # get current time from system (epoch time)
my $threshold = 900;

$year -= 1900;
$mon -= 1;
my ($hour, $min, $sec) = split /:/ => $time;

my $nsec = chop($sec);
my $mtime = timegm($sec, $min, $hour, $mday, $mon, $year);
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However, without your actual input file, I'm just guessing. Adding warnings and following the advice in Basic debugging checklist will help you track down the issue. Alternatively, I could help you debug if you posted your input file, wrapped in <code> tags.

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