I'm not sure one can put perl code in the conf file - dont forget that Log::Log4perl can also parse xml and java - but if it doesn't perhaps your best bet is to dynamically generate the conf file itself - put a line like

log4perl.appender.LOG.filename=MARKER
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and have your script do a

s/MARKER/gen_log_name()/e;
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or such like - sorry for the brevity - she who must be obeyed just summoned me home.