I usually use

#!/usr/bin/env perl

Is there any difference from using "which"?

From the man page of "which" I would expect exactly the same behaviour:

Which takes one or more arguments. For each of its arguments it prints to stdout the full path of the executables that would have been executed when this argument had been entered at the shell prompt. It does this by searching for an executable or script in the directories listed in the environment variable PATH using the same algorithm as bash(1).

Update
difference is run vs. print to stdout.