Do you use login like: $sftp = Net::SFTP::->new($host, %login)? I haven't ever used Net::SFTP, but from a quick glace at the source, it looks like ssh_args => is supposed to be there and is supposed to be followed by an ref to an array containing extra parameters to put in the call to Net::SSH::Perl::->new. It's hard to see how what you have would work:

sub new {
    my $class = shift;
    my $sftp = bless { }, $class;
    $sftp->{host} = shift;
    $sftp->init(@_);
}

sub init {
    my $sftp = shift;
    my %param = @_;
    $sftp->{debug} = delete $param{debug};
    $param{ssh_args} ||= [];

    $sftp->{_msg_id} = 0;

    my $ssh = Net::SSH::Perl->new($sftp->{host}, protocol => 2,
        debug => $sftp->{debug}, @{ $param{ssh_args} });
...
[download]

with no further use of @_ in init(). It looks like you want something like (completely untested):

$sftp = Net::SFTP::->new($host,
                         user => $username,
                         password => $password,
                         ssh_args => [
                            options => [
                               "StrickHostKeyChecking no"
                            ]
                         ]
                        );
[download]

Which is basically what the doc says, except that both modules say reference to a list where they mean reference to an array.