This is a fine opportunity to use List::Util's reduce function:

    use List::Util qw( reduce );

    sub array_to_nested_hash {
        reduce { scalar {$b,$a} } undef, reverse @_;
    }
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Now, to test our subroutine:

    use Data::Dumper;
    $Data::Dumper::Terse  = 1;
    $Data::Dumper::Indent = 0;

    print Dumper( array_to_nested_hash(1..5) ), "\n";
    # {'1' => {'2' => {'3' => {'4' => {'5' => undef}}}}}
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Cheers,
Tom