What is the difference between:   open (MPRINT, "$mconn|") or die $!; and:   open (MPRINT, "|$mconn") or die $!;

The first executes the program in $mconn and you can read its output through the filehandle MPRINT.

The second allows you to write some input through the filehandle MPRINT which the program in $mconn can access on its STDIN.