Remember to use WikiFormatting please.
bzip2 is a compression library. MPlayer presumably uses it to compress/decompress things, so it can't be removed. You can infer this by noticing that bzip2 is listed as a library dependency of mplayer-devel, and confirm it by examining mplayer-devel's files with the otool -L command.
gettext is an internationalization library. It is used to provide user interface strings in different languages. I don't know whether MPlayer itself uses it, but its dependencies do, so it cannot be removed. MacPorts policy is to provide full-featured software, so we don't provide an option to turn natural language support off; see comment:ticket:32304:2.
perl is a scripting language. I'm not sure in what capacity it is used, but it is listed as a library dependency of glib2 (which means it is used by glib2 at build time and at runtime), and glib2 is a library dependency of liboil, which is a library dependency of mplayer-devel, so it can't be removed.
