Using the ternary operator:

my $rain = 1;
my %Weather = (
    'precipitation' => ($rain == 0) ? 'dry' : 'wet',
);
[download]

Or even:

my $rain = 1;
my %Weather = (
    'precipitation' => $rain ? 'wet' : 'dry',
);
[download]

The way to use the if/elsif construct:

my $precipitation;
if ($rain == 0) {
    $precipitation = 'dry';
}
elsif ($rain == 1) {
    $precipitation = 'wet';
}
[download]

See perldoc for Conditional Operator and Compound Statements, and also strict and warnings