my %meta_char = (
       '%'  => sub { '%' },
        c   => sub { $chan },
        m   => sub { $in_message },
        P   => sub { $$ },
        p   => \&_get_package,
        s   => \&_get_subroutine,
        t   => \&_get_timedate,
    );

($out_message = $log_format) =~ s/%(.)/$meta_char{$1}->()/ge;