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;
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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.

Smylers