use IPC::Run3 'run3';
my $stdin = "password\n";
my @extcmd = ('cat');
run3 \@extcmd, \$stdin, \my $stdout, \my $stderr
    or die "run3 failed";
$? and die "run3: \$?=$?";
print "stdout: <<$stdout>>\n";
print "stderr: <<$stderr>>\n";