You seem to be arbitrarily splitting on spaces. This will break unless there's a clear space either side of the +. A better option would be as follows. 
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";

[download]
The basic premise being to scoop up and ignore (push onto output) anything in <maths>, or scope up as much that can easily be determined not to be in <maths> (ie no angle brackets) and parse that before putting it on the output.

In reply to Re^2: Regular Expression Doubt by reasonablekeith
in thread Regular Expression Doubt by Anonymous Monk

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