my $text = '1 &plus; 2 <br> <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";

__END__
1 &thinsp;&plus;&thinsp; 2 br> <maths> &plus; dfdf</maths>