Yeah, that's one advantage of the implementation of this type of iterator that is provided in Algorithm::Loops:

use Algorithm::Loops qw< NestedLoops >;

my $listIter = NestedLoops(
    [ ( ['a'..'z','A'..'Z','_'] ) x 6 ],
    { OnlyWhen => 1 },
    # { OnlyWhen => sub { 3 <= @_ } },
);
my $strIter = sub {
    join '', $listIter->();
};
my $str;
while(  $str= $strIter->()  ) {
    print $str, $/;
}
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The OnlyWhen option lets you get all lengths or just pick which lengths you are interested in.

(I should add an option for wrapping the return value so I could build the string iterator without the extra step.)