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.

Even corion's regex will fail if there is a newline anywhere between the tags. This can easily be fixed with the /s modifier, but are there are probably more special cases yet to be found. Use a proven module.
Bill

Yes, you're right. I decided to go the saner route and use XML::Twig & Time::Piece. ;-)

perl -pe 's/<Last-Modified>(.*)<\/Last-Modified>/`date -d \"$1\" \"+%s\"`/ge&&s/\n//' mydoc.xml

Let's hope that mydoc.xml never contains something like this:

NEVER pass unverified user input to a shell (i.e. ``, qx(), system $string, exec $string), and NEVER without quoting problematic characters. And don't hope that all shells have the same quoting rules. They differ even for the common unix shells (see e.g. http://www.in-ulm.de/~mascheck/various/), and it gets very much worse as soon as you leave unix. CMD.EXE and especially COMMAND.COM just cause mental illness. If you want to stay sane, try to avoid the shell. Stay with perl modules, or at least use the list forms of system and exec. perlipc has some examples for safely replacing `` and qx().

Alexander