my $ws = qr/\s+/;

sub get_token {
   my ($self) = @_;
   for ($self->{buf}) {
      s/^$ws//;
      if (length() == 0) {
         my $fh = $self->{fh};
         return [ TOK_EOF ] if eof($fh);
         $_ .= <$fh>;
         redo;
      }

      s/^([a-zA-Z][a-zA-Z0-9_]*)// && return [ TOK_IDENT, $1 ];
      ...
   }
}