With binmode, what you have would work.

perl -i.bak -pe "BEGIN { binmode STDOUT } s/(?=\[)/\r\n/g" file
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Without binmode, \n gets transformed to CR+LF, so \r\n become CR+CR+LF.

perl -i.bak -pe "s/(?=\[)/\n/g" file
[download]

Update: Escaped [ as noticed by Tanktalus.