Untested:

my $logged_in = eval { $ftp->login($un, $pw); 1; };

if (! $logged_in){
    if ($@ =~ /login incorrect/i){
        # do stuff, perhaps keep asking user for un/pw and
        # re-attempt login in a loop
    }
    else {
        # might not be a login error...
        # handle different exception
    }
}
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