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my $extcmd = 'cat';
open my $cmd_pipe, '|-', $extcmd or die "open $extcmd: $!";
print $cmd_pipe "password\n";
close $cmd_pipe or die "close $extcmd: ".($! ? $! : "\$?=$?");

use IPC::Run3 'run3';
my $stdin = "password\n";
...
$? and die "run3: \$?=$?";
print "stdout: <<$stdout>>\n";
print "stderr: <<$stderr>>\n";