my $string = '00110011111110111111111111111110';
my $str_to_replace;
for ($string=~/(1+)/g) {
    my $temp = $1;
    $str_to_replace = $temp if length($tmp) > length($str_to_replace);
}
if ($str_to_replace) {
    my $repl = '2' x length($str_to_replace);
    $string =~ s/$str_to_replace/$repl/;
}