What ikegami is referring to, I believe, is that the method is expecting a file in FASTA format, which means that sequences are always preceded by a "description line" having the format

>foo bar baz and whatever else
[download]

This line holds identifying info for the following sequence. Therefore, your tied array class would have to prepend to each array element some surrogate for this line (e.g. at minimum ">\n", or something more informative, such as ">$n\n", where $n is a class variable holding the current position in the input array).

Another complication is that the FASTA format requires that all lines be at most 80 characters long. So your READLINE method would have to do a bit more processing and bookkeeping than simply spitting out the next array element. No biggie.

Alternatively, the module in question may be able to handle some other file format that is more easily mimicked with a tied array than FASTA is.