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

my @t;
my $x = -1;
local *_ = \$text;
s!(<maths>.*?</maths>)!$x++;push@t, $1;"##XAHJASH $x##"!egs;
s/&plus;/&thinsp;&plus;&thinsp;/g;
s/##XAHJASH (\d+)##/$t[$1]/ge;
print