The documentation says:
identity_files: (...) The value of this argument should be a reference to an array of strings, each string identifying the location of an identity file.

So try:

my $files = [ '/usr/home/username/.ssh/identity' ];
my $ssh = Net::SSH::Perl->new("host", 
                              "identity_files" => $files);
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alex pleiner <alex@zeitform.de>
zeitform Internet Dienste