hyperref with implicit=false gives a warning

The setting of option pdftitle is used in \PDF@FinishDoc. Usually it is called at the output of the first page, but with option implicit it is done via \AtBeginDocument.

Thus, either you can use your call of \AtBeginDocument before package hyperref is loaded, or package etoolbox helps with hook \AtEndPreamble:

\documentclass{article}
\usepackage[implicit=false]{hyperref}
\usepackage{etoolbox}
\AtEndPreamble{\hypersetup{pdftitle=mytitle}}
\begin{document}
Hello world
\end{document}