You can accomplish your desired result using Backreferences. With your sample input, I would use the code: 

#!/usr/bin/perl
use strict;
use warnings;

$_ = 'testing 1234567 ........ aaaaaaaasssssss __________ ++++++++++ -
+-------- testing testing ';

s/((.)\2\2)\2*/$1/g;
print;

[download]

to output

testing 1234567 ... aaasss ___ +++ --- testing testing

YAPE::Regex::Explain describes the regex as: 

  (                        group and capture to \1:
----------------------------------------------------------------------
    (                        group and capture to \2:
----------------------------------------------------------------------
      .                        any character except \n
----------------------------------------------------------------------
    )                        end of \2
----------------------------------------------------------------------
    \2                       what was matched by capture \2
----------------------------------------------------------------------
    \2                       what was matched by capture \2
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \2*                      what was matched by capture \2 (0 or more
                           times (matching the most amount possible))

[download]

Note I have kept the third 'testing' in the string, which seems to follow spec but not your example.

Update: As AnomalousMonk points out below, swapping \2* to \2+ is more rigorously correct and avoids some unnecessary no-ops. However, I didn't think this particular optimization was likely to have sufficient impact to warrant changing the post content. For a cleaner version, you can also use s/(.)\1{3,}/$1 x 3/eg; (or s/(.)\1{$n,}/$1 x $n/eg;).

In reply to Re: Supress similar chars in the string by kennethk
in thread Supress similar chars in the string by Lana

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