Sounds like your system doesn't know how to resolve the port for service ssh. Try this:

my $ssh = Net::SSH::Perl->new($host, port => 22);   # ssh

Update: alternatively, teach your system about ssh to avoid this problem in the future (at least on this machine). Edit your %WINDIR%\system32\drivers\etc\services, and add this line, preferably just before the one for telnet to keep things sorted:

ssh                22/tcp                           #Secure Shell