Correct:
@charset "iso-8859-7";    FAILPASS

Wrong Capitalization:
@CHARSET "iso-8859-7";    PASSFAIL
@ChArSeT "iso-8859-7";    PASSFAIL
@cHaRsEt "iso-8859-7";    PASSFAIL

Wrong Whitespace:
@charset"iso-8859-7";     PASSFAIL
@charset  "iso-8859-7";   PASSFAIL
@charset\9"iso-8859-7";   PASSFAIL
@charset\A"iso-8859-7";   PASSFAIL
@charset\C"iso-8859-7";   PASSFAIL
@charset\D"iso-8859-7";   PASSFAIL
@charset "iso-8859-7" ;   PASSFAIL
 @charset "iso-8859-7";   PASSFAIL

Wrong Quote character:
@charset 'iso-8859-7';    PASSFAIL
@charset "iso-8859-7';    PASSFAIL
@charset 'iso-8859-7";    PASSFAIL

Wrong Semicolon:
@charset "iso-8859-7"     PASSFAIL