Every day I realize I have more to learn. Your code is much more elegant than my recursive solution.

use warnings;
use strict;
use Data::Dumper;

my @lists = ([qw/Truck Car Bike/],
             [qw/Truck Car Scooter/],
             [qw/Truck Trailer Skates/]);

my %tree;

for my $list (@lists) {
    to_tree(\%tree, $list);
}

print Dumper(\%tree);

sub to_tree {
    my ($head, $list) = @_;
    my $node = shift @$list;

    return unless defined $node;

    $head->{$node} ||= {};
    to_tree($head->{$node}, $list);
}
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