# $special_characters_escaped = backslash( $source_string );
sub backslash ($) {
	local $_ = ( defined $_[0] ? $_[0] : '' );
	# Preserve only printable ASCII characters other than \, ", $, and @
	s/([^\x20\x21\x24\x25-\x39\x41-\x5b\x5d-\x7e])/\\$Backslashed{$1}/gs;
	return $_;
}