Ok,
Forget RCS/CVS part. Let me rephrase

$cmd needs some input from user and it will until user enters something. It will first print something on STDOUT and then wait for STDIN. You can imagine like "Press Y or N" and now user has to make choice.
If I use open3, I am not seeing STDOUT message and program is waiting
While using "open3" will STDIN of parent be connected to STDIN of child - if not how to achieve this.
Sample code:
hello.pl
#!/usr/intel/bin/perl
print "Press Y or N:";
$l = <stdin>;
print "You have entered $l\n";
test.pl
#!/usr/intel/bin/perl
use IPC::Open2;
##can be replaced with open3 later
#open3 (*HIS_IN, *HIS_OUT, 0, "hello");
open2 (*OUT, *IN, "hello.pl");
print <OUT>;
## the above should have the same functionality as system --HOW ??
##system ("hello.pl");