use strict;
use warnings;



local $/ = "\n##"; #record separator

while (<DATA>) {
    chomp;

    #print "\n<<<$_>>>\n"; # check input

    my ($head,$rest) = /^ (.*?) \n (.*) $/xs;

    # print record only if $rest contains alphanumerics
    print "##$_" if $rest =~ /\w/;
}




__DATA__
## This gets stripped



## This doesn't

Because it contains this line



## More headers

blah blah


## This should not get stripped, but it does



### This should prevent it from getting stripped, but it doesn't

stuff



#### This should also not get stripped



##### But it does