How can i perform a substitution (s/foo/bar/) safely, without treating either value as a regular expression?

Sed is not the right tool for this. At best, it will be an escaping nightmare, and extremely prone to bugs.

First, what are we performing the substitution on? If it's a string, it can be done very simply with a parameter expansion. 

var='some string'
echo "${var//some/another}"

This is discussed in more detail in Faq #100.

If it's a file or stream, things get a bit trickier. One way to accomplish this would be to combine the previous method with Faq #1. 

# file
while IFS= read -r line; do
  printf '%s\n' "${line//foo/bar}"
done < file

# command output
while IFS= read -r line; do
  printf '%s\n' "${line//foo/bar}"
done < <(my_command)

my_command | while IFS= read -r line; do
  printf '%s\n' "${line//foo/bar}"
done

The second of the two command examples there creates a subshell. See Faq #24 for more information on that.

Both of the above examples print to stdout. Neither actually edits the file in place. Of course this could be resolved with something like: 

while IFS= read -r line; do
  printf '%s\n' "${line//foo/bar}"
done < file > new_file && mv new_file file