A "Definitions" section, like a glossary, can be an extremely useful resource for readers. It doesn't follow, however, that readers will (a) read and thoroughly master the content in such a section before proceeding with the rest of the document, or (b) appreciate having to page back multiple times through numerous intervening pages (if the document is long) to find the appropriate definition each time a term makes its debut in the main section of the document. Some portion of your readers may not be aware of the "Definitions" section at all, if they are reading only parts of the document or if they forget that they bypassed the "Definitions" section at the outset.
As Edwin Ashworth notes in his comment, different publishers have different rules for handling subsequent appearances of terms that appear initially in a "Definitions" section—and those rules tend not to be negotiable. Nevertheless, it makes sense to put yourself in the reader's shoes and judge whether the convenience of including a same-page definition when a term first arises in the main body of a document constitutes a significant benefit to the reader, notwithstanding the existence of the reference section.
It's a judgment call, of course, and it depends to a large extent on the length of the document, the complexity of the terminology you're using, and the level of familiarity that you can confidently expect a typical reader of your document to have with that terminology. But making the right call will amply repay the effort you put into analyzing the situation.
Identifying a title by a single word from it is by no means a recent innovation, nor does it seem to be tied to the length of the original wording. Consider The Tragedy of King Lear, cited most often as King Lear, but frequently referred to simply as Lear, as in the opening paragraph of Charles Jennens, "The Tragedy of King Lear, as Lately Published, Vindicated" (1772):
As the new edition of Shakespeare's Lear was attacked in a very rude and scandalous manner, by the Critical Reviewers ; and the patron, the editor, and another person who had no concern therein (but Whom they judged to be the editor) were treated in very abusive and scurrilous terms, by this society of gentlemen, as in their title-page they are pleased to stile themselves ; it was thought proper, upon presenting another play to the public, to vindicate the said edition of Lear from the base aspersions and misrepresentations which these Drawcansirs [that is, broadswords, as opposed to rapiers] in criticism had cast upon it.
Likewise, we find The Posthumous Papers of the Pickwick Club commonly reduced to The Pickwick Papers, and from there sometimes shortened further to Pickwick, as in Joseph Miller, Reading Narrative (1998), excerpted in The Novel: An Anthology of Criticism and Theory 1900–2000 (2009):
The latter two relations, that between author and narrator, that between text and critic, are articulated with special clarity in the passage from Pickwick Papers. In Pickwick, moreover, the way all three examples exploit properties more salient in written, not spoken, language is made explicit.
And some authors regularly refer to A Streetcar Named Desire as Streetcar, as in Philip Kolin, Tennessee Williams: A Guide to Research and Performance (1998):
Cohn usefully comments on the symbology of the names in Streetcar, while Kolin explicates the mythic and gaming allusions behind Jax Beer ("Why Stanley"). Kolin also explores the network of paper signifiers in and underneath the script in Streetcar, including poetry, legal documents, and artifacts, and concludes that for Williams paper is "both script and Scripture" [citation omitted].
Of course, these short forms are helped by the fact that the works they refer to are unlikely to be misidentified by their readers—but that is surely true, too, of the short form Bergeron once you have properly introduced the complete title Harrison Bergeron to your readers. I concur with Lore Sjöberg that Bergeron, being a more memorable identifier than Harrison, would be a better choice for the short-form title.
I would caution you, however, that some readers may react unfavorably to your use of a short form of the title, as Cerberus and Edwin Ashworth do in the comments beneath your question. Also, some titles resist reduction to one word more vigorously than others do; thus for example, having shortened Moby-Dick; or, The Whale to Moby-Dick, I would strongly advise against shortening it further to either Moby or Dick.
Ultimately, your safest bet is probably to follow Cerberus's advice and refer to the title by its full name or (for variety's sake) by a descriptive term such as "the story."
Best Answer
Taking your example, I'd use a more memorable abbreviation, and I'd make the first mention part of a full sentence:
There are plenty of options -- as always in technical writing, you should prioritise clarity. Clarity is rarely served by text that reads as legalese (except presumably to lawyers) so henceforth referred to or even worse hereinafter referred to is of no help.