system( 1, "first" );  # Won't wait.
system( "second" );    # Will wait.
[download]

This is could be very portable though it also still appears to be mostly undocumented. ):

Update: I posted this long ago and later learned that system(1,...) was never allowed to work except on Windows (and OS/2, for which it was originally written). Any documentation on it appears to be relegated to perlport.