You can think of any regular expression in term of some nested for loops.

Please, consider:

for my $i (0 .. $eos) {
    for my $j ($min .. $max) {
        last if $j > $eos-$i;
        print substr($str, $i, $j),"\n";
    }
}
[download]

The following variables specifies the meaning of "ABCDEF" =~ /(\w{2,}?)(?{print "$1\n"})(?!)/;

my $min = 2;                      # minimum match length
my $max = 32767;                  # maximum match length
my $str = "ABCDEF";
my $eos = length($str);
[download]

Using the same nested for loops, we have another specification for "ABCDEF" =~ /([A-Z]{3})(?{print "$1\n"})(?!)/;

my $min = 3;                      # minimum match length
my $max = 3;                      # maximum match length
my $str = "ABCDEF";
my $eos = length($str);
[download]

By executing the code, it gives us the same output as the corresponding regular expressions do. I hope this will give you a basic intuition on how backtracking works.