=~ is not related to the ~ operator; the assignment variant of ~ would be ~=, except that ~ is a unary operator and so has no assignment variant.

What you want is to select everything before the first \s, and assign that to $message_id; use a match, not a substitution, to do that:

($message_id) = $message =~ /^(\S*)/;
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(Note the parentheses around $message_id that make it a list assignment; you need to do the assignmentmatch in list context to have it return anything besides success/failure.)