Hi
after a lot of trials i have succeeded in installing the latest rakudo from source on windows xp, the above problem is the same, it seems prompt function adds additional letter to the right of the one digit number ,
i can get around such behaviour with this code until the problem resolved in future releases:

use v6;

my $num = prompt("Please write a number : ");
if $num.chars == 2 {$num = substr $num, 0, 1;}
say $num;
$num +=3;
say $num;
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regards