sub combinations {
    my ($num, $arr) = @_;

    return sub { return }
        if $num == 0 or $num > @$arr;

    my @pick;

    return sub {
        return @$arr[ @pick = ( 0 .. $num - 1 ) ]
            unless @pick;
        
        my $i = $#pick;
        $i-- until $i < 0 or $pick[$i]++ < @$arr - $num + $i;
        return if $i < 0;

        @pick[$i .. $#pick] = $pick[$i] .. $#$arr;
        
        return @$arr[@pick];
    };
}

##</code><code>##

my $iter = combinations( 3 => ['a' .. 'f'] );

while ( my @c = $iter->() ) {
    print "@c\n";
}