I think typeglobs are the way to go if you want to figure out at run time what method to override.

my $package_method = 'Foo::Bar::baz';
my $orig = \&($package_method};
no warnings;
*{$package_method} = sub { pre(); $orig->(@_); post() };
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You can also do this:

package Foo::Bar;
sub baz { print "I am the original!\n" }

package main;
sub Foo::Bar::baz { print "I am the new baz!\n" }

Foo::Bar::baz();
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Or you can toss in your own definition between a couple package lines anywhere you want. You can also build up a string like the above at run time and wrap it in an eval whenever you're ready.

Note that these various methods can have subtle differences when it comes to caller, and maybe some other stuff.

Thank you. This seems to be the best way to do it. may I trouble you for a small illustrative example? Here is what I want to do. I have a Package written by someone else and I want to override one of the methods in that Package without touching its source. However I want to do it in a way that even if that package calls, I want it to use my version. Thank you once again

uhm, isn't the documentation Sub::Install clear enough?
to give you an example i would just copy&paste from there anyway...