My guess is you'll need to decode it at some point too. This all very similar to run length encoding, though I think RLE encodes even single character occurrences. So here's short snippet to encode/decode which you can modify to suit your needs: 
use strict;

sub encode {
    s/((.)\2+)/(length $1) . $2/eg;
    $_;
}

sub decode {
    $_ = shift;
    my @list;
    while (/((\d+)?(.))/g) {
        push @list, [$2,$3];
    }
    join '', map { (defined $_->[0]) ? $_->[1] x $_->[0] : $_->[1]; } 
+@list;
}

while (<DATA>) {
    print;
    my $enc = encode($_);
    my $dec = decode($enc);
    print $enc;
    print $dec;
}

__DATA__
XYZAAAAAAAADEFAAcdAA

[download]
Which gives the following output: 
XYZAAAAAAAADEFAAcdAA

XYZA8DEFA2cdA2

XYZAAAAAAAADEFAAcdAA

[download]

---
s;;:<).>|\;\;_>?\\^0<|=!]=,|{\$/.'>|<?.|/"&?=#!>%\$|#/\$%{};;y;,'} -/:-@[-`{-};,'}`-{/" -;;s;;$_;see;
Warning: Any code posted by tuxz0r is untested, unless otherwise stated, and is used at your own risk.

In reply to Re: Compressing a text file using count of continous characters by tuxz0r
in thread Compressing a text file using count of continous characters by nirvana4ol

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.