*old_foo = *foo;
require 'foo_killer.pl';
*foo = *old_foo;

{
    local *foo;
    require 'foo_killer.pl';
}

*old_foo = *foo{CODE};
require 'foo_killer.pl';
*foo = *old_foo{CODE};

package Foo;
use strict;
use warnings;

use Exporter;
our @ISA = qw(Exporter);
our @EXPORT_OK = qw(foo bar $evil $global $variables);

do "foo_killer.pl"
     or die "Failed to load foo_killer.pl: ".($@ || $!);
1;

my $foo_killer = __FILE__; # $LIB/Foo.pm
$foo_killer =~ s,(/+[^/]+),legacy,; # $LIB/legacy
$foo_killer .= "foo_killer.pl"; # $LIB/legacy/foo_killer.pl
# now do $foo_killer;