use strict;

my $prev_length = 0;
my $threshold = 40;
my $first_line = 1;

while (<DATA>) {
    chomp;
    # s/^\s+//g;
    # s/\s+$//g;

    my $curr_length = length($_);
    if ($prev_length > $threshold && $curr_length > 0) {
        print " ";
    } elsif ($first_line) {
        $first_line = 0;
    } else {
        print "\n";
    }

    print "$_";
    $prev_length = length($_);
}
print "\n";

__DATA__
This is a longish line whose linebreak should be removed
so that this line runs together with it.

The preceding double linebreak should be conserved.

This little line
and this one
should stay separated.

##</code><code>##

This is a longish line whose linebreak should be removed so that this line runs together with it.

The preceding double linebreak should be conserved.

This little line
and this one
should stay separated.