Obligatory Algorithm::Loops version:

use Algorithm::Loops qw( NestedLoops );
my $minLen= 2;
my $maxLen= 4;
my $iter= NestedLoops(
    [ ( [qw( A T C G )] ) x $maxLen ],
    { OnlyWhen => sub { $minLen <= @_ && @_ <= $maxLen } },
);
my @bases;
while(  @bases= $iter->()  ) {
    print @bases, $/;
}
[download]