Because it matches zero characters, the pattern would match at exactly the same starting position and for the same length more than once. An infinity of times, really. Perl detects this and causes the pattern not to match. so backtracking occurs.

You can see the same thing in this simpler example:

$ perl -le'"aaaaab" =~ /[aA](?{ print pos })[bB]/'
1
2
3
4
5
[download]

Or better yet:

$ perl -le'"aaaaac" =~ /[aA](?{ print pos })[bB]/'
1
2
3
4
5
[download]

The expression in (?{ }) needs to be backtrackable.

$ perl -le'"aaaaab" =~ /[aA](?{ pos })[bB](?{ print $^R })/'
5

$ perl -le'"aaaaac" =~ /[aA](?{ pos })[bB](?{ print $^R })/'

$
[download]

Unfortunately, that can't be done in your case.