More general: how to convert number n to base b.

1. b^k ≤ n; find k; k = floor( log_bn )
2. t * b^k ≤ n; find t; t = floor( n/b^k )
3. put sign nr. t from your base at end of resulting string, there must be exactly b signs in the base array.
4. set n = n - t * b^k
5. go to step 1 or finish if n = 0

in other words:

sub decimalToX {
    my $number = shift or return $_[0];
    my $base = @_;
    my $logbase = log $base;
    my $string = '';

    my $power = int(log($number)/$logbase);

    while($number){

        my $f = $base ** $power;
        my $times = int($number/$f);

        $string .= $_[$times];
        $number -= $times * $f;

    } continue {
        --$power
    }

    $string . $_[0] x ($power+1)
}
print decimaltoX( 5 , qw/0 1/ ); # 101
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