See :help substitute of :help substitute().
It is the counterpart of the substitute command (See :help :substitute).
substitute({expr}, {pat}, {sub}, {flags}) *substitute()*
The result is a String, which is a copy of {expr}, in which
the first match of {pat} is replaced with {sub}. This works
like the ":substitute" command (without any flags). But the
matching with {pat} is always done like the 'magic' option is
set and 'cpoptions' is empty (to make scripts portable).
'ignorecase' is still relevant. 'smartcase' is not used.
See |string-match| for how {pat} is used.
And a "~" in {sub} is not replaced with the previous {sub}.
Note that some codes in {sub} have a special meaning
|sub-replace-special|. For example, to replace something with
"\n" (two characters), use "\\\\n" or '\\n'.
When {pat} does not match in {expr}, {expr} is returned
unmodified.
When {flags} is "g", all matches of {pat} in {expr} are
replaced. Otherwise {flags} should be "".
Example: >
:let &path = substitute(&path, ",\\=[^,]*$", "", "")
This removes the last component of the 'path' option.
:echo substitute("testing", ".*", "\\U\\0", "")
results in "TESTING".
In your example I guess that
let l:bar = substitute(l:bar, "world", "kitten", "") should work