|Description:|| Perl one-liner to remove from a text file all lines that contain only whitespace.
Other Ways To Do It (mostly not one-liners) can be found at the thread "How do I remove blank lines from text files?".
Hmmm... personally, I like Ovid's example below better than mine. It *does* seem more directly recognizable.
perl -i.bak -ne 'print unless /^\s+$/' file.txt