What happened when you tried it?

use strict;

package Foo;

sub new { my( $c, $v ) = @_; return bless { val => $v }, $c; }

sub wubble { print "Wubble: ", shift()->{'val'}, "\n"; };

package main;

my $f = Foo->new( "abc" );

$f->wubble;

eval { $f->flop; };
if( $@ ) {
  print "Problem: $@\n";
}

eval q{sub Foo::flop { print "Flop: ", scalar reverse( shift()->{'val'
+} ), "\n"; }};

eval { $f->flop; };
if( $@ ) {
  print "Problem: $@\n";
}

exit 0;

__END__
Wubble: abc
Problem: Can't locate object method "flop" via package "Foo" at /tmp/s
+quonk line 15.

Flop: cba
[download]

(Of course if you're serious then Moose and Class::MOP would be less hacky ways to do it.)