Well, use eval(). For example, for my HTTP-Daemon I need to redefine a socket method that make impossible to share the same socket port with multiple childs on Win32, so I use this:

# Fix a bug on sockname of IO::Socket, that block the app for multi ch
+ilds with
# the same server opened. Or for a GLOB blessed in a package different
+ than
# IO::Socket, since the socket of the server is blessed to HTTP::Daemo
+n! (Bug saw only on Win32).

if ( $^O =~ /(?:^ms|win|dos)/i ) {
  eval(q`
    package HTTP::Daemon ;
    sub sockname {
      #getsockname($_[0]);
      getsockname( fileno($_[0]) );
    }
  `);
}
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But I think that the best option is to extend the class (like robartes said), since this doesn't change the method for all the mod_perl scripts (let's say that you run different scripts inside mod_perl):

package Apache::FakeRequest::ViceRaid;
use base qw(Apache::FakeRequest);

  sub lookup_uri { ... }
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Graciliano M. P.
"The creativity is the expression of the liberty".