It's calling the method hash on the variable $sql in the main package passing the first argument the sub was given. $Foo::bar means to access the scalar variable $bar in the symbol table of package Foo; if the package name is omitted it's the same as if you'd specified $main (so your example is really just $main::sql->hash( $_[0] )).

This is all covered in perlmod in the first few paragraphs (well the :: and package symbol table stuff is at least).