if(...){...}else{...} is considered a statement rather than an expression. Any expression can be used as a statement, but many statements cannot be used as an expression.

In a list (which is what a hash is initialized with), the list items are expected to be given as expressions, not statements.

The ternary operator is an expression, which is why that works.

A do{...} block allows you to provide a block of statements and wrap it up into an expression:

my %weather = (
   precipitation => do {
      if ($rain == 0) {
         "dry";
      }
      else {
         "wet";
      }
   },
);
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A couple other examples along these lines:

my ($x, $y, $z) = ( ... );
...
my %weather = (
    'precip'     => get_precip_function($x, $y, $z),
    'wind_speed' => sub { ... }->($z, $y, $x),
    ...,
    );
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