You need to dereference $files[($ii-1)/3]{map1]. It contains a reference to an array, so

  print "map1 : [",
        join( ',', @{$files[(($ii-1)/3)]{map1}}),
        "]\n";
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will fix it. I changed join's first argument to be a string, and replaced concatenation with a print list.

After Compline,
Zaxo