The basic idea behind the code is: Foo->bar(@_) is really a nice way of saying Foo::bar("Foo", @_) and $f->bar(@_) is Foo::bar($f, @_). After that, ref $self returns false for the string "Foo" (as it is not a reference) but the class name for the object.

I don't think it's a bad approach at all, but it will break existing subroutine-using code unless you somehow sniff what type the first argument is.

Since your functional interface already requires passing a reference on every call, I don't actually see why anyone would prefer to use it. Personally, if your module is not in too wide use yet, I would choose to retire it and tell the people depending on your module to change their code.

I agree with you. According to your idea, users don't need to pass a reference subroutines on every call:

use Blosxom::Header;

# procedural interface
my $value = Blosxom::Header->get($key);
my $bool  = Blosxom::Header->exists($key);
Blosxom::Header->set($key => $value);
Blosxom::Header->remove($key);

# OO interface
my $h     = Blosxom::Header->new();
my $value = $h->get($key);
my $bool  = $h->exists($key);
$h->set($key => $value);
$h->remove($key);
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Nobody uses my module except for me :) But I belive this module should exist.

Update:

I noticed that it's not necessary to implement OO interface any more in this case.

On the other hand, there's nothing that forbids you from creating wrappers which you can export. Adding to my previous code:

@EXPORT_OK = qw/get_bar/;

sub get_bar {
    my $var = shift; # your $blosxom::header hashref
    bar($var, @_);
}
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If a module looks as follows:

package Foo;
@EXPORT_OK = qw(get_foo);

sub new {
    # constructer
}

sub get {
    # for OO interface
}

sub get_foo {
    # for procedural interface
}

1;
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users may write as follows:

$o = Foo->new();
$o->get($key);     # of cource, $value
$o->get_foo($key); # what's this?
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If users choose OO interface, they shouldn't be able to call 'get_foo' method. How can I solve this problem?

If users choose OO interface, they shouldn't be able to call 'get_foo' method. How can I solve this problem?

I'm not very well versed in Perl's OO, so someone please correct me if I'm wrong, but I don't think you can forbid users from calling it. The only thing you can do is throw an error on unexpected arguments.

(There is a convention that a class's private methods' names begin with the underscore character, though.)