my @x = qw(a b c d e f);
my $n = 4;

my $x = (1<<$n)-1;
while( $x < 1<<@x ){
    my $i = $x<<1;
    print join", ",(grep{($i>>=1)&1}@x),"\n";
    $i = $x&(~$x>>1);
    $i &= -$i;
    $x += $i--;
    if( $i &= $x ){ 
        $x -= $i;
        $i /= ($i&-$i);
        $x += $i;
    }
}
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