If it were my code, I'd probably write it the way moritz has it, but if you're extra paranoid, you might do something like this:

($out_message = $log_format) =~ s{%(.)}{
    my $m = $meta_char{$1};
    (ref sub {} eq ref $m) ? $m->() : $1
}ge;
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That will withstand missing elements as well as elements that exist but don't have a code reference in them.