I think the examples given are a bit misleading; the key part is the definition of the PassphraseCallback argument in the docs, in particular the last part:

    In either case, the callback routine should return the
    passphrase, a scalar string.

The various examples sometimes leave off the final call to _prompt(), and since it is an undocumented internal routine it isn't obvious that this is doing the guts of the work: showing a prompt, setting noecho, getting the text from the user's input, and returning the resulting string.

I'd suggest looking at the code for _prompt() in the module (it's in the top-level Crypt/OpenPGP.pm) to see what needs doing, and then (probably) avoid using ths undocumented interface directly by copying what you need out of there.

Here's some similar code I use in a less critical situation, which copes with the possibility that Term::Readkey isn't installed by letting the password be entered unhidden:

sub getpass {
    my $prompt = shift;
    local $| = 1;
    print "$prompt: ";
    eval { require Term::ReadKey };
    my $haveterm = !$@;
    Term::ReadKey::ReadMode(2) if $haveterm;    # turn on noecho
    my $pass = <STDIN>;
    chomp $pass;
    Term::ReadKey::ReadMode(0) if $haveterm;    # restore
    print "\n";
    return $pass;
}
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