You are only accessing the element of @obj, never of $obj.

The JSON structure contains a reference to an Array of Hashes already, so assining that to an array (@obj) makes things more complicated.

If you assign the decoded JSON result to a scalar, $obj, then things Just Work:

my $obj= from_json( $data );
print $obj->[0]->{acc_id};
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Using variables with the same name that differ only in the sigil is confusing.