use strict;

my $line = '$x += 2; $y = $x**2';

$line =~ s/([^\sa-zA-Z0-9])/'&#'.ord($1).';'/ge;

# or if you don't want to encode underscores
$line =~ s/([^\s\w])/'&#'.ord($1).';'/ge;

print "$line\n";

##</code><code>##

s/([^ a-zA-Z0-9])/'&#'.ord($1).';'/ge   # a single, literal space