I often like using sprintf for it instead of variable concatentation .. note also using single quotes for the string literals means you don't have to have the escaping backslashes...

my $message = sprintf '<event_id="ALTER_%d_%d" source="APP">', $tier, 
+$code1;
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Also, to OP: I second blazar's comments, and would also add to them that your assignment of $severity with the two if statements can simply be written as:

$severity = $code1 ? 1 : 5;
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Though, what's the point of $severity ? It doesn't seem to be used...