timelocal cannot handle the ambiguous hour (from 1 to 2am at the end of daylight savings time), because it has no $isdst parameter. Example:

use strict;
use Time::Local;
use POSIX;

my ($time,$time2,$sec,$min,$hour,$mday,$mon,$year,$isdst);
my ($wday, $yday, $hm);
$sec = 59;
$min = 59;
$hour = 0;
$mon = 9;
$mday = 28;
$year = 2001;
$time = timelocal($sec,$min,$hour,$mday,$mon,$year);
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime($tim
+e);

$hm = sprintf("%02d/%02d/%04d %02d:%02d:%02d",$mon+1,$mday,$year+1900,
+$hour,$min,$sec);
print "Local time: $hm and DST is $isdst\n";

# Skip ahead one hour.
$time += 3600;

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime($tim
+e);
$hm = sprintf("%02d/%02d/%04d %02d:%02d:%02d",$mon+1,$mday,$year+1900,
+$hour,$min,$sec);
print "Local time: $hm and DST is $isdst\n";
$time2 = timelocal($sec,$min,$hour,$mday,$mon,$year);
if ($time == $time2) {
   print "timelocal conversion correct -- timestamp is $time\n";
} else {
   print "timelocal conversion error -- $time converted to $time2\n";
  ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime($t
+ime2);
   $hm = sprintf("%02d/%02d/%04d %02d:%02d:%02d",$mon+1,$mday,$year+19
+00,$hour,$min,$sec);
   print "(Local time: $hm and DST is $isdst)\n";
}

$time2 = POSIX::mktime($sec,$min,$hour,$mday,$mon,$year,0,0,1);
if ($time == $time2) {
   print "mktime conversion correct -- timestamp is $time\n";
} else {
   print "mktime conversion error -- $time converted to $time2\n";
  ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime($t
+ime2);
   $hm = sprintf("%02d/%02d/%04d %02d:%02d:%02d",$mon+1,$mday,$year+19
+00,$hour,$min,$sec);
   print "(Local time: $hm and DST is $isdst)\n";
}

# -------- end of example -----------
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This prints:

Local time: 10/28/2001 00:59:59 and DST is 1
Local time: 10/28/2001 01:59:59 and DST is 1
timelocal conversion error -- 1004255999 converted to 1004259599
(Local time: 10/28/2001 01:59:59 and DST is 0)
mktime conversion correct -- timestamp is 1004255999
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