the documentation says:

* identity_files 
...
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.
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So if your identity is in id_rsa.pub maybe you should let the object know that during instantiation:

my $ssh = Net::SSH::Perl->new(
    $host, 
    identity_files => ["$ENV{HOME}/.ssh/id_rsa.pub"],
  );
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