#!/usr/bin/perl
use strict;

doStuff(); # no need for &
foo();

sub doStuff {
   # want to read the file as a single string....
   local $/; # if you don't set it to anything, it is undefined
   open TMP, "<somefile.txt";
   my $file = <TMP>; # $file since you are putting the whole file into a scalar, not a list
   close TMP;
   # do more stuff with that file...
}
sub foo {
   ## here i would want to read the file line-by-line
   
   open TMP, "<otherfile.txt";
   my @file = <TMP>;
   close TMP;
}