#! perl -slw
use strict;

permIt(split '', 'ABC---');

sub _next
{
    my ($v) =  @_;
    my  $t  = ($v | ($v - 1)) + 1;

    return ($t | (((($t & -$t) / ($v & -$v)) >> 1) - 1));
}

sub permIt
{
    my $l = @_;
    my @chars = grep $_ ne '-', @_;
    my $n = my $s = (1 << @chars) - 1;

    do
    {
        my $bits = pack 'V', $n;
        my @copy = @chars;
        my $str  = join '',
                   map { vec( $bits, $_, 1 ) ? shift( @copy ) : '-' } 0 .. $l - 1;
        print $str;
        $n = _next($n);

    } until ($n > ($s << ($l - @chars)));
}