How to do it depends partly on how you want to handle the case

Item1--Item2
Item1--Item2--Item3
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Should the first Item2 be something different from the second because it's a leaf?

In my solution below, I'm assuming that the answer is "no".

my %h;
while (<>)
{
    chomp;
    my @v = split;
    my $h = \%h;
    for ( @v )
    {
        $h->{$_} ||= {};
        $h = $h->{$_};
    }
}
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