I was personally puzzled at what $^T is for a moment, though I knew about $^I. So the simple form would be

mkdir bak/ ; perl -pi'bak/*' -e 's/foo/bar/g' file1 file2 file3 ...
[download]

Or

perl -pi'bak/*' -e 'BEGIN { mkdir($^I =~ m!(.*/)!) } s/foo/bar/g' file
+1 file2 file3 ...
[download]

Or maybe even

perl -MFile::Path -pi'bak/*' -e 'BEGIN { mkpath [ $^I =~ m!(.*/)! ] } 
+s/foo/bar/g' file1 file2 file3 ...
[download]

*grin*

Makeshifts last the longest.