In your data, the third (and fourth and fifth) digits from the right are the same. And this is what you are capturing. You will not get any match where the captured numbers are different.

Second thing, I think you should probably make your spaces optional if you do not have spaces at the beginning or at the end of your lines (or perhaps use a word boundary anchor).

With these two points in mind:

  DB<8> $_ = "105 106 107 108 109 110 111 112 113 213 214";

  DB<9> s[\s?(\d+)\d\d\K\s(?=(\d+)\d\d\s?)]{$1 + 1 == $2 ? "\n" : ' '}
+ge;

  DB<10> x $_
0  '105 106 107 108 109 110 111 112 113
213 214'
  DB<11>
[download]

I also think that this:

$1 + 1 == $2 ? "\n" : ' '

is going to fail on sequence jumps, e.g. on data looking like this:

$_ = qw /105 106 107 108 109 110 111 112 113 313 314/;

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