FWIW - this is the code I finally settled on ..

my %opt = ( 
      DEBUG                => 0,
      OTHER_UNRELATED_OPT  => "Other values",
      # snip 8< --- more params here ...
    LOCALHOST            => $ENV{HOSTNAME} || $ENV{COMPUTERNAME} || $E
+NV{HOST},
      DATEFIELD            => sub{ local $_={};
                                @$_{qw|sec min hour mday mon year wday
+ yday isdst|} =localtime(time);
                                $_->{year}+=1900;
                                $_->{mon}+=1;
                                $_->{yyyymmdd} = sprintf "%04d-%02d-%0
+2d", @$_{qw|year mon mday|};
                                $_->{hhmm}     = sprintf "%02d:%02d", 
+     @$_{qw|hour min|};
                                return $_} # end of anon sub 
                                ->(), # Call the sub 
      REMOTE_PARAMS        => "",
   );
[download]

Thanks everyone !