a.pm

package a;

sub foo {
  warn "foo";
}

sub rebind {
  no warnings "redefine";
  eval q{delete $a::{foo}};
  eval q{*foo = sub { warn "bar" }};
}

sub go {
  foo();
  rebind();
  foo();
}

1;

gistfile2.txt

$ perl -Ma -e'a::go(); a::foo(); eval q{a::foo()}'
foo at a.pm line 4.
foo at a.pm line 4.
foo at a.pm line 4.
bar at (eval 2) line 1.