Perhaps you want $resp->{xmlref}{Details}[0]{ReleaseDate}? The arrow means that you want to follow a reference, and the square brackets find array elements.

One way you can explore this kind of structure is by using Data::Dumper to show the results of a partial access:

  print Dumper( $resp );
  print Dumper( $resp->{xmlref} );
  print Dumper( $resp->{xmlref}->{Details} );
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Check out the perlref manpage for more information on the reference syntax.