While this is correct, I would like to add that any values that exist in both %A and %B are overwritten by the one in %B. For example:

my %A = (a => "A", b => "B");
my %B = (b => "A");

my %C = (%A, %B);

print "$_ => $C{$_}\n" foreach keys %C;
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Output:
a => A
b => A