try...

$args{identity_files} = [ $identity ];   # takes ARRAY
$args{protocol} = '2,1';                 # try v2 first
[download]

works for me...

use Net::SSH::Perl;
$ssh =
  Net::SSH::Perl->new( "rtrfoo",
    { identity_files => ["$ENV{HOME}/.ssh/id_dsa"], protocol => "2" } 
+);
$ssh->login("myself");
print "@{[$ssh->cmd(q(show version))]}"
__END__
Hostname: rtrfoo
Model: m20
JUNOS Base OS boot [5.6R2.4]
JUNOS Base OS Software Suite [5.6R2.4]
...
[download]