I don't know exactly how Sametime expects to get it's login, can you be more specific what the program does? Does it output a prompt, like Login:? Usually to interact with a program, the Expect module is usually recommended, google for perl Expect for sample code.
in reply to Calling an executable and entering login credentials
In a very simple situation, you may be able to get by with IPC::Open3, which opens the program, and gives you pipes to write to, and read from, the executable.
A simple example:
# you would run Sametime here, instead of bash
my $pid=open3(\*IN,\*OUT, \*ERR , '/bin/bash');
# set \*ERR to 0 to send STDERR to STDOUT
# in your case, print the login name here
my $cmd = 'date';
#send cmd to bash
print IN "$cmd\n";
my $result = <OUT>;