Your new method is being called as a class method, therefore the class is the first argument rather than the second. ie, the class is in $self.

sub new {
    my ($self,$class)=@_;
....
our $cache=Fiets::Cache->new();
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my ($class) = @_;
# or
# my $class = shift;
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And your use of base is um, just wrong. 'use base' is meant to be set at a package level, and when you use it, you don't need the 'use *package*' line.
I know 'use base' does an implicit 'require', but sometimes ...
Is your cachedir set once per program, and you want to inherit from Cache::FileCache or Cache::NullCache based on it per program run, or can it change per instance of an object? I'm not going to guess at what you want there.
That part of the program is working fine, the 'getvar' function checks %ENV and a config-file for clues on what to use. The whole program just uses 1 instance of Fiets::Cache.

Thanks a lot, I totally missed the $self there!

Are you sure you're inheritance chain is being set up properly? use and base are compile-time directives, which means that it doesn't matter where they appear in your code, they'll be executed before your code runs. If it's running as expected, that is, you're able to find the inherited methods when you call them on an instance of your subclass, then I would think you've got some multiple inheritance going on. If you truly want those to be set at runtime, I'd use require and manually adjust the @ISA array.

---

"The dead do not recognize context" -- Kai, Lexx

All subroutines (or functions or methods or whatever you were taught to call them in CS101) in perl return a value. You can explicitely return a value by using a return statement, or, the last line is returned. For instance, in the function:

use strict;
use warnings;

sub foo {
  1;
}

print foo, "\n";
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You would see an output of 1. Now, you never explicitely returned a value of 1, but because it was the last thing on the line, Perl tried to do what you meant by returning 1. However, Perl is not psychic. So, when you see most constructors ending with:

  bless $self, $class;
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$self is not modified. An object is created when bless is evaluated. The following would do the same thing:

  my $object = bless $self, $class;
  return $object;
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Consult perldoc bless for more information. So, this:

    bless ($self,$class);
    return $self;
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should read:

    return bless ($self, $class);
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Also, note that the first element of @_ is the class name, not the second. So:

    my ($self,$class)=@_;
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Should really be the other way around, i.e.:

    my ($class, $self) = @_;
    # and add a check that we're not copying
    # for good measure
    die ("This isn't a copy constructor") if (ref $class);
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And unless you know what you're doing, you shouldn't use anything but a hashref for $self. I think you might want to google for @ISA and read up a little bit on inheritance.

However, Perl is not psychic. So, when you see most constructors ending with:   bless $self, $class; $self is not modified.

This is wrong, $self is modified. Try e.g. this:

sub foo {
    my $self = {};
    bless $self, 'Bar';
    return $self;
}

print "isa Bar.\n" if foo()->isa('Bar');
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-- Hofmator

I'm no perl expert, so I'm guessing here. Is your definition of $class correct? You don't seem to be passing it as an argument of $cache=Fiets::Cache->new();

In other words, shouldn't class be defined as... $class=ref($self) ??