\documentclass[11pt,a4paper]{article}

\begin{document}

This is just a simple example. Compiling it with the following \texttt{.latexmkrc} should print ``This line should be printed'' before starting \texttt{pdflatex}.

\begin{verbatim}
$pdflatex = 'internal internalexample %R %O %S';

sub internalexample {
    my @args = @_;
    print "This line should be printed\n";
    return system("pdflatex @args");
}
\end{verbatim}

\end{document}