Eh, if you go that way, it could be made much more efficient. No need for the s/// - you're matching any + already:

my $output = "";
while ($text =~ m{\G(?:&plus;|(<maths>.*?<\/maths>|[^&<]+|.))}gs) {
    $output .= $1 || "&thinsp;&plus;&thinsp;";
}
print $output, "\n";
[download]