#! 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)));
}
[download]

Output:

17:18 >perl 760_SoPW.pl
ABC---
AB-C--
A-BC--
-ABC--
AB--C-
A-B-C-
-AB-C-
A--BC-
-A-BC-
--ABC-
AB---C
A-B--C
-AB--C
A--B-C
-A-B-C
--AB-C
A---BC
-A--BC
--A-BC
---ABC

17:18 >perl -v

This is perl 5, version 18, subversion 1 (v5.18.1) built for MSWin32-x
+86-multi-thread-64int
...
[download]