Keeping in mind that this sounds truly evil, though it does sound a bit like Damian's Hook::LexWrap (though I don't think that redefines any subs, just defines new methods with pre and post processing, but you may want to look at that instead/anyway), and you're better off using dragonchild's suggestions of utilizing ISA and SUPER in an orthodox manner if at all possible:

use strict;
use warnings;

package Bar;

sub f1 {
 print "there @_\n";
}

package Foo;

for (keys %Bar::) {
 if (exists &{$Bar::{$_}}) {
  no warnings 'redefine';
  my $func = \&{$Bar::{$_}};
  $Bar::{$_} = sub {
     print "hello ";
     goto &$func;
  };
 }
}

package main;

Bar::f1("Bob");
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