my $text = '1&plus;2<maths> &plus; dfdf</maths>';

my $output = "";
while ($text =~ m/((<maths>.*?<\/maths>)|([^<]*))/gs) {
	if ($2) {
		$output .= $2;
	} else {
		my $segment = $3;
		$segment =~ s/&plus;/&thinsp;&plus;&thinsp;/g;
		$output .= $segment;
	}
}
print $output . "\n";