# suppose this is the input string
my $str_in;
# and this is the output
my $str_out;

my $char;
my $char_count;

my $len = length $str_in;

for (my $i=0; $i<$len; $i++) {
    my $curr = substr $str_in,$i,1;
 
    if (!defined $char) {
        # initialize state
        $char = $curr;
        $char_count = 1;
    }
    else {
        if ($curr eq $char) {
            # current char is same with previous ones
            if ($char_count>=3) {
                $char_count++;
            }
            else {
                $str_out .= $curr;
                $char_count++;
            }
        }
        else {
            # we found a different char
            $char = $curr;
            $char_count = 1;
            $str_out .= $curr;
        }
    }
}