moritz's regex does work. Calling the replace operator (s///) once, produces the desired result. The solution fails because the replace operator is potentially called more than once by the while loop. I think the easiest solution is to remove the while loop. 
#!/usr/bin/perl
use strict;
use warnings;

my $toTranslate = "333333333333333333333333333333333333333333333333333
+333333333333333333333333333333333333333333333333333333333333333333233
+333333322333333333233333333333333333333333333333333323333333322333333
+332333333333";

$toTranslate =~ s/((?:3{4})+)/"[" . length($1) . "]"/ge;

print $toTranslate,"\n";

[download]
The documentation notes that the global replace is not progressive like the match operator (in scalar context). That is only applicable to successive calls to the operators, not for the completion of a single call.

In reply to Re: Regex exact pattern match problem! by eye
in thread Regex exact pattern match problem! by Jts

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