$str= 'no change <these spaces need replacing> not these <these do>';
$str =~ s{(<[^>]*?>)}
        {
         my $x=$1;
         $x=~s/ /_/g;
         $x;
        }egx;
print $str


##</code><code>##

($_='jjjuuusssttt annootthheer
     pppeeerrrlll haaaccckkeer')=~y/a-z//s;print;