use IPC::Open2;

local (*RD, *WR);
my $child = open2(\*RD, \*WR, 'passwd');
# RD and WR are now pipes connected to STDOUT/STDERR and
# STDIN respectively of the child process. $child contains
# the PID.

my $prompt = <RD>;      # not caring what the prompt is, here
print WR "password\n";
$prompt = <WR>          # prompted again
print WR "password\n";