Perl solution to the Vietnamese pesky snake

vietnamese_snake.pl

#!/usr/bin/perl                                                                                      
#                                                                                                    
# Solution to the Vietnamese pesky snake in perl :)                                                  
# Calculates all 128 solutions                                                                       
# http://www.theguardian.com/science/alexs-adventures-in-numberland/2015/may/21/how-to-solve-the-maths-puzzle-for-vietnamese-eight-year-olds-that-stumped-parents-and-teachers                           
#                                                                                                    
# function: a+(13*b/c) + d + (12*e) - f - 11 + (g*h/i)-10;                                           

# Setup the numbers to insert                                                                        
@a = qw(1 2 3 4 5 6 7 8 9);

# For counting solutions                                                                             
$count = 0;

# Start permutating                                                                                  
&perm(0);

# Using easy backtracking                                                                            
sub perm {
    my ($k) = @_;

    my $len = $#a+1;
    if ($k==($#a+1)) {
        my $sum = $a[0]+(13*$a[1]/$a[2])+$a[3]+(12*$a[4])-$a[5]-11+($a[6]*$a[7]/$a[8])-10;
        if ($sum==66) {
            print ++$count.": ";
            for my $i (0..$#a) {
                print $a[$i]." ";
            }
            print "\n";
        }
    } else {
        for (my $i=$k; $i<=$#a;$i++) {
            my $t = $a[$k];
            $a[$k] = $a[$i];
            $a[$i] = $t;
            &perm($k+1);
            $t = $a[$k];
            $a[$k] = $a[$i];
            $a[$i] = $t;
        }
    }
}