Face your fears. Embrace AUTOLOAD.

Here is one approach:

package Frob;

sub new {
    my $pkg = shift;
    bless {}, $pkg
}

sub customize {
    my ($self, $method, $code) = @_;
    $self->{_custom}->{$method} = $code;
}

sub AUTOLOAD {
    my $self = shift;
    my $method = $AUTOLOAD;
    $method =~ s/.*:://;
    if ( exists $self->{_custom}->{$method} ) {
        $self->{_custom}->{$method}->(@_)
    } else {
      die "no code for '$method'"; 
    }
}

package main;

$|++;        # so as not to suffer from buffering

my $frob = new Frob();

$frob->customize('foo', sub { print "foo @_!\n" });
$frob->foo(42);    # should work
$frob->bar(42);    # should fail
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