QM's solution is clever. Here is a prosaic method that works for a known maximum number of keys:

my ($key, $val) = split /=/;
my @keys = split /:/, $key;

if (@keys == 1) {
   $hash{ $key[0] } = $value;
}
elsif (@keys == 2) {
   $hash{ $key[0] }{ $key[1] } = $value;
}
# etc.
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