In truth, I like using logical operators for this sort of thing, and personally find them very readable. Benefit of a C background.

The thing that bothers me with reading your code is your API. It is emergent from the logic and not very intuitive. You are implementing a class with a single closure which takes zero or more arguments.

• $fifo->() returns the next value from the queue, adjusting the state of $fifo.
• $fifo->(undef) falls all the way through, returning a defined false and not altering state.
• $fifo->($foo,@bar), with $foo defined, enqueues the elements of the arg list, returning the total length of the queue including those elements which have already been removed. This is unexpected, to me.
• $fifo->(undef,'cmd'), for 'cmd' one of (reset|all|ordered|dump) either rewinds the queue (returning 0), or returns a peek at all its values.
• $fifo->(undef,undef,@bar) triggers an undefined value warning.

Note that each instance of a uFIFO carries an interpreter around with it.

How about an implementation as a module? Make Uniqueue.pm define a data class with a constructor, a $fifo->enqueue( @foo ) acting like push, and a $fifo->next() which acts like either shift or a nondestructive iterator, depending on what behavior you want. Other methods like 'all' or 'reset' then can be defined as methods also.

I've coded a Uniqueue.pm in terms of shift. If there's interest, I'll send it to the catacombs.

After Compline,
Zaxo