foreach my $i( reverse 1..$len){
        my $x = 10 * $a[$i-1] + $q * $i;
        $a[$i-1] = $x % (2 * $i - 1);
        $q = int($x / (2 * $i - 1));
    }

[download]
not sure if it's better or worse, but I'd change that to 
    foreach my $i( reverse 0 .. $len-1){
        my $x = 10 * $a[$i] + $q * ($i+1);
        my $divisor = 2 * $i + 1;
        $a[$i] = $x % $divisor;
        $q = int($x / $divisor);
    }

[download]
not for efficiency, but for (perhaps) easier understanding

In reply to Re^3: porting C code to Perl -- solved by soonix
in thread porting C code to Perl by Discipulus

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