https://stackoverflow.com/questions/33800565/how-to-store-linux-command-output-into-a-variable-in-puppet

If you want to execute any command on Puppet Master server you can use inline_template function with ERB template inside and Ruby code for execute shell command:

$password = "12345"
$secretvalue = inline_template("<%= `/bin/echo ${password} | /usr/bin/base64` %>")
notify { "STDOUT: ${secretvalue}": }

P.S. If you just want to encode string to Base64 format you can import puppetlabs-stdlib module and use base64 function from it:

$secretvalue = base64('encode', $password)