condenser.pl

#!/usr/bin/perl -w
use 5.010;
use strict;
use warnings;
use autodie;

while (<>) {

    # This requires at least two characters to fire, because we're searching
    # for a character, followed by repeats ('+') of that character.
    #
    # We're doing string substitution (rather than building a new string),
    # so if something doesn't match, we just leave it alone. 

    s{
        (?<string>     # The whole string of repeated characters.
            (?<char>.) # The first character
            \g{char}+  # The same character, as many times we can.
        )
    }{$+{char} . length($+{string} ) }exg; # Replace with character and length of string.

    print;
}