The way that I have done this in the past is to create my own class that inherits from Apache::SOAP and incorporates a new constructor that includes the call to the serializer function.

For example:

package My::SOAP;


use Apache::SOAP;

use strict;
use vars qw( @ISA $VERSION );


#   This class inherits from the Apache::SOAP class and is intended to
+ provide
#   functionality relating to the SOAP server itself, rather than the 
+exposed
#   web services.

@ISA = qw( SOAP::Transport::HTTP::Apache );


my $server = __PACKAGE__->new;


sub new
{
    my $self = shift;
    unless( ref $self )
    {
        $self = __PACKAGE__->SUPER::new( @_ );
        $self->serializer( My::SOAP::Serialiser->new );


        #   The following on_action handler is intended to make this w
+eb service .NET
        #   friendly by modifying the SOAPAction header to fit the for
+m of URI#method
        #   (the form preferred by SOAP::Lite) where the SOAPAction he
+ader is of the
        #   form URI/method (that employed by the .NET framework).

        $self->on_action(
                sub
                {
                    ( my $action = shift ) =~ s/^("?)(.*)\1$/$2/;
                    if( $action &&
                            $action ne join( '#', @_ ) &&
                            $action ne join( '/', @_ ) &&
                            ( substr( $_[0], -1, 1 ) ne '/' ||
                                    $action ne join( '', @_ )))
                    {
                        $self->action( join '#', @_ );
                    }
                }
        );
    }
    return $self;
}


1;


__END__
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(This code, although slightly modified to protect the innocent, is from an Apache::SOAP server implementation that was developed for interface from .NET solution components and shows one way to implement your own serialiser within Apache::SOAP derived solutions).