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use strict;
use warnings;
...

$t2 =~ s{((?:3{4})+)}{ qq{[@{ [ length $1 ] }]} }eg;
print qq{Without either\n$t2\n@{ [ q{=} x 50 ] }\n};

Sequences of 3s:-
>33335<
...
Without either
[33332]3337
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