This is just a CSS selector
$dom->find('div.abstract-content > p')->each
Here we are doing a search to find each div, with class abstract-content which has a child <p> tag.
MDN has a nice section on, CSS Selectors, and plenty of other resources too. To quote Child combinator:
"The child combinator (>) is placed between two CSS selectors. It matches only those elements matched by the second selector that are the direct children of elements matched by the first."
The MDN is linked to in the basics section of the overall Mojo documentation, https://mojolicious.org/perldoc. If you have any other questions let me know.