I'm back playing with Net::SSH2 and running into trouble again. My code is still the very very simple

my $ssh2 = Net::SSH2->new() ;
$ssh2->connect(HOST) or $ssh2->die_with_error ;
$ssh2->check_hostkey(tofu => HOSTKEY) 
    or $ssh2->die_with_error ;
$ssh2->auth_publickey(USER, PUBLICKEY, PRIVATEKEY)
    or $ssh2->die_with_error ;
$ssh2->auth_ok() ;
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Here's what's happening: I have a real SSH client that uses a single SSH keyprofile for two different unix systems. I've double checked and the and authorized.keys files on both servers are identical. I have the two constants: HOST and USER. when I changed the HOST and USER to be the new system and the username to use on that system, I get

Username/PublicKey combination invalid (-18 LIBSSH2_ERROR_PUBLICKEY_UN
+RECOGNIZED
) at D:\Perl\ondreamhost.pl line 28.
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I have no idea what it is trying to tell me. What do usernames have to do with SSH keys?