Due to:
IE interprets the URL for the behavior property relative to the source HTML document, rather than relative to the CSS file like every other CSS property
(more info), the url is kept in a variable, to be easily changed according to the project:
$pie-path: "/myproject/PIE.htc";
No prefix needed unless 2.1 Android and below and 3.2 iOS and below or CSS3PIE for IE8 and below so this mixin is not really needed anymore – more info