#!/tmp/bleadperl/bin/perl -w 

use re 'eval';

my $re;
my $len = 20;

$_ = "this is a long string, and I want to insert newlines every 20 chars";

$re = qr{
  (??{ '\b\w{1,' . ($len - ($+[0] - $-[0])) . '}\b' })
  (?(?{ ($+[0] - $-[0]) < $len })
    (??{ '\W{0,' . ($len - ($+[0] - $-[0])) . '}' })
    (?(?{ ($+[0] - $-[0]) < $len })
      (?(?=
        (??{ '\b\w{1,' . ($len - ($+[0] - $-[0])) . '}\b' }))
        (??{ $re })
      )
    )
  )
}x;

s{($re)}{$1\n}xg;

print;