If it is an OO-module, then you can simply subclass it. Say your Parser is called Foo::Parser. Then you can create a class My::Foo::Parser and override all methods you like. Looks basically like this:

package My::Foo::Parser;
use base (Foo::Parser);

sub new
{
  my $class = shift;
  my $self = $class->SUPER::new (@_);
  
  #do your own constructor_code here

  return $self;
} 

sub method
{
  my $self = shift;
  my $complexHash = $self->SUPER::method();
  #do your own stuff here

  return $complexHash;
}
[download]

See the various OO-Tutorials for more about subclassing.

Hope that helps.