Perl 6, 13 bytes

put sum lines 

Try it

Perl 6, 13 bytes

say sum lines 

Try it

Explanation

• lines() returns a list of lines from $*IN or $*ARGFILES a “magic” command-line input handle.
• sum(…) was added to Perl 6 to allow [+] List to be optimized for Positionals that can calculate their sum without generating all of their values like 1..100000
  (I just thought sum was just too cute here to use [+] like I normally would)
• say(…) call the .gist method on its input, and prints it with an additional newline.