Depending on how line-oriented your XML is, that regular expression might find none, one or all matches.

See perlre on "greedy" - .* will match as much as possible, including an intervening </Last-Modified> tag if possible.

There are two approaches to fixing this. The easy approach is to read the whole file into memory and do the replacement directly:

perl -0777 -le 's!<Last-Modified>(.*?)</Last-Modified>!<Last-Modified>
+`date ...`</Last-Modified>!ge' mydoc.xml
[download]

The saner way is to use one of the XML parsing modules, for example XML::Twig.

Also, I would look at Time::Piece or Time::Local for doing the time conversion instead of shelling out to the date program for every replacement.