my $output = "";
while ($text =~ m/((<maths>.*?<\/maths>)|&plus;|.)/gs) {
    if ($2) {
        $output .= $1;
    } else {
        my $segment = $1;
        $segment =~ s/&plus;/&thinsp;&plus;&thinsp;/g;
        $output .= $segment;
    }
}
print $output . "\n";