Here's my implementation -

$text =~ s/(?<=\b)\n(?=\n\b)//g;
[download]

Consider the string below -

This is line1\n
\n
line3\n
\n
\n
[download]

What the regex will do is to get rid of the '\n' at the end of line1, which is followed by another \n and a word boundary.

This solution is similar to qq's answer above, but with subtle differences. qq's solution will replace two consecutive \n's with a single \n, while my solution will get rid of the first \n.