dave_the_m is right, there's no way to do that. This is clearly stated in perlsub:

To unambiguously refer to the built-in form, precede the built-in name with the special package qualifier CORE::. For example, saying CORE::open() always refers to the built-in open(), even if the current package has imported some other subroutine called &open() from elsewhere. Even though it looks like a regular function call, it isn't: you can't take a reference to it, such as the incorrect \&CORE::open might appear to produce.

You can create a wrapper to one, but you need to go through hoops to do it:

$substr = sub {
   if (@_ == 2) {
      return substr($_[0], $_[1]);
   } elsif (@_ == 3) {
      return substr($_[0], $_[1], $_[2]);
   } elsif (@_ == 4) {
      return substr($_[0], $_[1], $_[2], $_[3]);
   } else {
      require Carp;
      Carp::croak("Bad arguments for substr");
   }
};

print($substr->('abc', 1), "\n");
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substr(@_)
and
substr($_[0], $_[1], $_[2], $_[3]) (unconditionally)
won't work, because builtins have prototypes, and the
&substr
trick to bypass the prototype doesn't work on builtins.
(&CORE::substr doesn't work either.)

Dave.