The "esxcli --server <servername> --username <my username> --password <my password>" is how vSphere CLI application connects to the ESXi Host, and then it runs the "vicfg-snmp.pl --server <servername> -c public" against the host.
I have also tried using the same vicfg-snmp.pl command while logged into the host using putty, and get the same thing.. "-sh: vicfg-snmp.pl: not found"
So far I have tried vSphere CLI, PowerGUI,and Putty (both telnet and SSH), all connect to the ESXi Host fine, but no matter the command I try and send to the host, the host tells me ii doesn't know the command.
Below is the sanitized output of the command that you recommended using Putty to the connect to the ESXi Host.
VMware offers supported, powerful system administration tools. Please
see www.vmware.com/go/sysadmintools for details.
The ESXi Shell can be disabled by an administrative user. See the
vSphere Security documentation for more information.
~ # vicfg-snmp.pl --server <server> --username <user> --password <password> --show
-sh: vicfg-snmp.pl: not found
~ #