As far as I know, the exec command does not launch the argument program in background mode. It launches the program in the foreground and never returns to the Perl program. But the Perl program really appears to end when the argument program completes. This is an example on Unix of an exec command taking a bit of time to execute:

$ time perl -e 'print "start program. \n"; exec ("time ls -lR /usr | w
+c");'
start program.
  36311  269214 1917392

real    0m8.933s
user    0m1.309s
sys     0m3.883s

real    0m9.012s
user    0m1.339s
sys     0m3.929s
[download]

The command passed to exec is counting all the files in the /usr directory and all its subdirectories. It takes a bit of time since there are more than 36000 entries. Under Unix, the time program measures the execution time. The first report states the time taken by the exec command. The second one gives the time taken by the Perl program. As you can see, the Perl program completed only after the exec command finished. No background process here. Under Unix, I could launch my command in the background this way:

$ time perl -e 'print "start program. \n"; exec ("time ls -lR /usr | w
+c &");'
start program.

real    0m0.090s
user    0m0.045s
sys     0m0.030s

Laurent ~
$   36311  269214 1917392

real    0m4.102s
user    0m1.200s
sys     0m2.963s
[download]

Here, the exec command is really launched in the background and the Perl program completes immediately (after 0.09 sec.) and the exec command prints the result and completes 4 seconds later. I do not know how you would do that under Windows, but you might have to launch an intermediate xxx.bat script that launches the background process and exits immediately.