Ah yes, that's known as an XY problem: You asked (first yourself, then the Perl Monks) about the content of PL_rsfp_filters (that's X), but your problem was understanding av_len() (Y). Good thing you mentioned av_len() in the body of your query. It might have been unanswerable otherwise.

if ( av_len(PL_rsfp_filters) > 0 )

Right, that checks for two or more filters. In view of the problem you had with the statement, it probably deserves a comment:

if ( av_len(PL_rsfp_filters) > 0 ) { 
    /* We only deal with a single filter */
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Update: Identified X and Y in the XY problem