Change:

  @files = sort { lc(${@$a}[0]) cmp lc(${@$b}[0]) } @$refdataarr ;
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To:

  @files = sort { lc $a->[0] cmp lc $b->[0] } @$refdataarr ;
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You are dereferencing the array reference in $a and $b as @$a and @$b and then trying to dereference that array as an array reference.