Your data structure shows that the def member is an array, as is tr. So you want something more like:

my $result = $decoded_json->{def}[0]{tr}[0]{text};
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Just remember that the square brackets show that you've got an array, and the curly braces are for a hash.

To access the syn elements, you should be able to do:

my $syn_array = $decoded_json->{def}[0]{tr}[0]{syn};
for my $rSyn (@$syn_array) {
   print "$rSyn->{text}, $rSyn->{pos}, $rSyn->{gen}\n";
}
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which should give you something like:

volta, noun, f
momento, noun, m
Time, noun, m
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...roboticus

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