From what I see at first glance, you seem to be missing a + on the last (\S) sequence.

I always debug my REs by deleting enough stuff such that it matches again. Then I add step by step the other stuff back in.

In your case, you might want to rewrite the code such that it logs "bad" lines into a separate file (untested code !):

Update :Shendal noted below that I didn't fix the problem with the RE, and that's correct. I have now modified the RE so that it should match :)

#!/usr/bin/perl -w
use strict;

my $filename = "access.log";
my ($client,$identuser,$authuser,$date,$tz,$method,$url,$protocol,$sta
+tus,$bytes,$refer,$platform,$extendedinfo)

open( FILE, "< $filename" ) or die ("Couldn't open $filename : !$\n" )
+;
while (<FILE>)  {
  if ( /^(\S+) (\S+) (\S+) \[(\S+) (\S+)\] "(\S+) (\S+) (\S+)" (\S+) (
+\S+) "(\S+)" "(\S+) (.*?)"$/ ) {

    ($client,$identuser,$authuser,$date,$tz,$method,$url,$protocol,$st
+atus,$bytes,$refer,$platform,$extendedinfo)
        = ($1,$2,$3,$4,$5,$6,$7,$8,$9,$10,$11,$12,$13);
  } else {
    print "Unmatched log entry : $_";
  };
};
close FILE;
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