This snippet removes all non-empty lines containing only whitespace, but I don't think that's particularly intuitive behaviour. If I want to remove all "blank" lines from a file, I probably want:

perl -i.bak -ne 'print if /\S/' file.txt
[download]

and if I want to get rid of all the lines with "useless" whitespace, I probably want to collapse them to empty lines:

perl -i.bak -ne 's/^\s+$//;print' file.txt
[download]

Update: Changed first one-liner to Ovid's for readability.

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:wq