Think about the following scenario:

There is the binary utility su, that you want to automate from a Perl script. For instance, you want to run  su -c "make install"

You write a script that just does system 'su -c "make install"'. But when you run it, it asks for a password, so you try...

open my $pipe, '| su -c "make install"';
print $pipe, "$password\n";
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But it keeps asking for the password because su is reading from /dev/tty.

What do you do then?