Keep Coding

Here is my naive approach:
# puppet/init.pp
$x = 'hello ' +
'goodbye'
This does not work. How does one concatenate strings in Puppet?

Keyword variable interpolation:
$value = "${one}${two}"
Source: http://docs.puppetlabs.com/puppet/4.3/reference/lang_variables.html#interpolation
Note that although it might work without the curly braces, you should always use them.

I use the construct where I put the values into an array an then 'join' them.
In this example my input is an array and after those have been joined with the ':2181,' the resulting value is again put into an array that is joined with an empty string as separator.
$zookeeperservers = [ 'node1.example.com', 'node2.example.com', 'node3.example.com' ]
$mesosZK = join([ "zk://" , join($zookeeperservers,':2181,') ,":2181/mesos" ],'')
resulting value of $mesosZK
zk://node1.example.com:2181,node2.example.com:2181,node3.example.com:2181/mesos

Another option not mentioned in other answers is using Puppet's sprintf() function, which functions identically to the Ruby function behind it. An example:
$x = sprintf('hello user %s', 'CoolUser')
Verified to work perfectly with puppet. As mentioned by chutz, this approach can also help you concatenate the output of functions.

The following worked for me.
puppet apply -e ' $y = "Hello" $z = "world" $x = "$y $z" notify { "$x": } '
notice: Hello world
notice: /Stage[main]//Notify[Hello world]/message: defined 'message' as 'Hello world'
notice: Finished catalog run in 0.04 seconds
The following works as well:
$abc = "def"
file { "/tmp/$abc":

You could use the join() function from puppetlabs-stdlib. I was thinking there should be a string concat function there, but I don't see it. It'd be easy to write one.