my %subst = (
      q|&| => q|&|,
      q|"| => q|"|,
      q|<| => q|&lt;|,
      q|>| => q|&gt;|,
      q|'| => q|&'|,
      q|`| => q|&`|,    
);
my $re = join '|', map quotemeta, keys %subst;
$data =~ s/($re)/$subst{$1}/g;