You're right about needing to use a reference to an array for assigning to a hash element, but that way of doing it can be rather wasteful. What you are doing with $Tex{$a} = [@bex] is copying the contents of @bex in to another array and then assign a reference to that new array to $Tex{$a}.

You can avoid the unneccesary copying by just taking a reference to the original array

if (!$Tex{$a}){
    $Tex{$a} = \@bex;
}
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Of course, this only works correctly in a loop if the array you are taking a reference to is scoped to the loop. If not, your method is correct.