What have you tried so far? The Net::SSH::Perl perldoc cites:

Net::SSH::Perl->new($host, %params)

To set up a new connection, call the new method, which connects to 
$host and returns a Net::SSH::Perl object.

new accepts the following named parameters in %params:

<snip>

o identity_files
A list of RSA/DSA identity files to be used in RSA/DSA
authentication.  The value of this argument should be a reference
to an array of strings, each string identifying the location of an
identity file. Each identity file will be tested against the server
until the client finds one that authenticates successfully.

If you don't provide this, RSA authentication defaults to using
$ENV{HOME}/.ssh/identity, and DSA authentication defaults to
$ENV{HOME}/.ssh/id_dsa.
[download]