I think it could be something like (not tested):

use Net::SSH::Perl;

my $host = 'host';
my $user = 'username';
my $pass = 'password';

my $string = 'Expression to append';
my $cmd = "echo $string >> /path/to/file"; 

my $ssh = Net::SSH::Perl->new($host);
$ssh->login($user, $pass);
my($stdout, $stderr, $exit) = $ssh->cmd($cmd);
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