The more idiomatic way of doing what you want is then:

echo 'http://dx.doi.org/'"$(pbpaste)"
The $(...) syntax is called command substitution. In short, it executes the commands enclosed in a new subshell, and substitutes the its stdout output to where the $(...) was invoked in the parent shell. So you would get, in effect:

echo 'http://dx.doi.org/'"rsif.2012.0125"