Looks like this has always been the case, with win32 perl-5.6.1

C:\> perl -MData::Dump -e " dd\@ARGV " perl -i".bak h "
["perl", "-i.bak h "]

C:\> perl -i".bak h "

Usage: *snip*
[download]

You might expect this to turn up some bug reports but it doesn't, see https://rt.perl.org/rt3/Public/Search/Simple.html?q=whitespace+-i