I've tinkered with the string lengths to get something more manageble, but you should be able to see what's happening.

See $INPUT_RECORD_SEPARATOR in perlvar for details of what $/ = \$max_length does. See open for a description of the effect of using a scalar reference instead of a filename.

use strict;

my $var = 'x' x 100;
my $max_length = 10;

$/ = \$max_length; # input record separator
$\ = "\n";         # output record separator
open my $FH, "<", \$var or die;
print while <$FH>;
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update: Added link to open