The solutions others have presented are way too complex.

while (my ($block) = $var =~ /(.{1,$max_length})/gs) {
   ...
}
[download]

Note that it's very bad to use $&. It slows down every regexp with no captures in your program.

Update: It a better way of doing the same thing as FunkyMonk's, johngg's, caelifer's, almut's and jwkrahn's code, but our code doesn't do exactly the same thing as yours.