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# Passing <<echo>> <<$$>> executes <<echo $$>>
$ ssh example.com echo '$$'
...
# Passing <<echo '$$'>> executes <<echo '$$'>>
$ ssh example.com 'echo '\''$$'\'''
$$

sub text_to_shell_lit(_) {
   return $_[0] if $_[0] =~ /^[a-zA-Z0-9_\-]+\z/;
...
  perl => ( '-e' => $perl_code );

backticks(ssh => ( '--', $target, $remote_cmd ));