The ssh implementation on the PIX is braindead. I'm having trouble with one of these beasts right now (not SSH-related). Cisco PIX 501 is such a piece of s*** that I can hardly refraing from tearing it to pieces with a sledgehammer!

You'll have a hard time connecting even from the command line. Try that first before you start coding in Perl. First, you need to generate a RSA host key on the beast, and enable SSH inbound connections. Make sure you replace the network address and mask with yours. At the serial console:

pixfirewall> enable
Password: 
pixfirewall# conf t
pixfirewall(config)#
pixfirewall(config)# ca generate rsa key 512
Keypair generation process begin.
Success.
pixfirewall(config)# ssh 192.168.1.0 255.255.255.0 inside
pixfirewall(config)# write mem
[download]

Then try connecting from the command line from a host matching the net address above (example using OpenSSH):

$ ssh -1 -c 3des -l pix 192.168.1.1
pix@192.168.1.1's password: <type 'cisco' for default>
Type help or '?' for a list of available commands.
pixfirewall>
[download]

I realize this reply is slightly OT, but I feel sympathy for the creatures who have to deal with these abominations. The suffering need to help each other! :)

Update: the OP writes:

I could manually establish ssh connection to it using the s/w 'putty' & could execute commands on it.

I was that mad at the beast that I didn't see that. Ignore.