use Text::Wrap;

use strict;
use warnings;

local $Text::Wrap::columns = 3;

my $f = "blah blah blah\nblah blah\n\nblah blah halb halb halb\n\nhalb
+ halb halb";

for my $line( split /\n\n/, $f ){
    print wrap('','',$line), "\n\n";
}
__END__
bl
ah
bl
ah
bl
ah
bl
ah
bl
ah

bl
ah
bl
ah
ha
lb
ha
lb
ha
lb

ha
lb
ha
lb
ha
lb
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