Hmm... changing this to either

perl -le '$,=":";print map {$_=reverse $_} split( /\./, (reverse "file
+name.ext"), 2)'
[download]

or

perl -le '$,=":";print map {(reverse $_)||""} split( /\./, (reverse "f
+ilename.ext"), 2)'
[download]

works... the statement I was working on prior to my posting of the question here was similar to the latter statement, which threw me off. Does || evaluate into $_?