gistfile1.txt

use strict;
use warnings;
use 5.020;
use IO::Pty;

my $pty = IO::Pty->new;
chomp (my $pass = readline);

my $tty = $pty->ttyname;

unless ( fork ) {
    $pty->make_slave_controlling_terminal;
    exec "gpg --use-agent --armor -b foo.txt";
}
say $pty $pass;
sleep 3;