Problem 11

#!/usr/bin/perl
use strict;
use warnings;
 
open INFILE, "<11.in";
 
my (@a, @tmp, $result);
my $max = 0;
 
while(<INFILE>){
        @tmp = split;
        push @a, [ @tmp ];
}
 
for my $r (0..$#a){
    for my $c (0..$#{$a[$r]}){
        #up
        if($r >= 3){
           $result = $a[$r][$c]*$a[$r][$c-1]*$a[$r][$c-2]*$a[$r][$c-3];
           $max = $result if $result > $max;
        }
        #left
        if($c >= 3){
           $result = $a[$r][$c]*$a[$r-1][$c]*$a[$r-2][$c]*$a[$r-3][$c];
           $max = $result if $result > $max;
        }
        #back diagonal
        if($c >= 3 && $r >= 3){
           $result = $a[$r][$c]*$a[$r-1][$c-1]*$a[$r-2][$c-2]*$a[$r-3][$c-3];
           $max = $result if $result > $max;
        }
        #forward diagonal
        if($r >= 3 && $c <= $#{$a[$r]}-3){
           $result = $a[$r][$c]*$a[$r-1][$c+1]*$a[$r-2][$c+2]*$a[$r-3][$c+3];
           $max = $result if $result > $max;
        }
    }
}
 
print "$max\n";
 
close INFILE;