my $message_id = $1 if $message =~ m/([\w\d]+)\s+.*/;
Putting my (a compile-time directive) with an if statement modifier (which can only take place at run-time) is confusing, and therefore deprecated.
Also, there's no point at all in having .* at the end of the regexp: since one of the things it would happily match is nothing, it isn't making any restrictions at all about what can follow the \s+, so is just adding noise.
Finally, the \w class already includes digits, so writing [\w\d] doesn't add anything.
Here's how I might have written this:
$message =~ /^(\w+)/ or die "Message '$message' doesn't start with an identifier.\n"; my $message_id = $1;
die might not be the most appropriate thing to do here; adjust as appropriate. But if you're adamant that $message could never not match the pattern, then a die test is better than nothing: if you're right, then it'll never happen, and if you're wrong then your assumption was wrong so there's a problem elsewhere and at least you found out about it.