Split each line on the divider (eg, split /\|/, ...) to get one or more fields. If there's one field, make sure a key with that value exists in some hash. If there's one field, push the second field's value onto the value (an array reference) associated with the first field (all this in the same hash mentioned before). If there are more levels, decide whether you want the third, etc fields to be sibling to the second value or children of it.

For example, for sibling-only behavior:

my %menu_structure;
for (@menu_specification) {
  my ($toplevel, @sublevel) = split /\|/;
  unless (@fields) {
    $menu_structure{$toplevel} = [];
  }
  else {
    push @{ $menu_structure{$toplevel} }, @sublevel;
  }
}
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