If you want to attach an array to a hash element, try

$groups[$gcount]{sub}=[@subgroup];
my $first_data =  $groups[0]{sub}[0];
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The  [] creates an anonmyous array that is attached as a reference to  $groups[$gcount]{sub}.

Update: My gues is that the code is parsing nested tags. The sub recursively calls itself on the string $newvalue found between a pair of tags. When there is no more text to parse, the recursion ends. While parsing, the intent is to build a tree of data enclosed in the tags. Thus the question on nested data structures.