I know that the SSH protocol defaults to 3des for encryption, if you are using blowfish by default, you may have to set that in the ssh_config. Net::SFTP passes that list into the Net::SSH::Perl sub that makes the connection;

# UNTESTED

my %args = ( 
    'user'       => $serverId, 
    'password'   => $serverPass, 
    'ssh_args'   => { 'cipher' => 'blowfish, },
);
my $stfp = Net::SFTP->new( $serverName, %args );
[download]

Update: ssh_config s/b ssh_args

You could also set the debug values for better messages;

my %args = ( 
    'user'       => $serverId, 
    'password'   => $serverPass, 
    'debug'      => 1,
);
my $stfp = Net::SFTP->new( $serverName, %args );
[download]

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Edit by castaway: Closed small tag in signature