After some search, I've found a cleaner solution wich use the % operator.
In your YAML file :
key : 'This is the foobar var : %{foobar}'
In your ruby code :
require 'yaml'
file = YAML.load_file('your_file.yml')
foobar = 'Hello World !'
content = file['key']
modified_content = content % { :foobar => foobar }
puts modified_content
And the output is :
This is the foobar var : Hello World !
As @jschorr said in the comment, you can also add multiple variable to the value in the Yaml file :
Yaml :
key : 'The foo var is %{foo} and the bar var is %{bar} !'
Ruby :
# ...
foo = 'FOO'
bar = 'BAR'
# ...
modified_content = content % { :foo => foo, :bar => bar }
Output :
The foo var is FOO and the bar var is BAR !