When you split the line, the result can be a list which contains less elements than you are expecting.

Thurs,July,5,2012,123,456,789 : here you'll obtain 7 elements, no more.

So, do not trust the input and don't forget to chomp your lines.

#!perl
use strict;
use warnings;

my @array=<DATA>;

chomp @array;
my $line;
my ($dow,$mon,$day,$year,$num1,$num2,$num3,$dow2,$mon2,$day2,$year2);
foreach $line (@array){
    ($dow,$mon,$day,$year,$num1,$num2,$num3,$dow2,$mon2,$day2,$year2) 
+= ((split /[,]/, $line),("")x11); 
    print "$dow2\n";
}    
    
__DATA__
Thurs,July,5,2012,123,456,789
Thurs,July,5,2012,123,456,789,Fri,July,6,2012
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