change "IBM & Microsoft" into "IBM & Microsoft"?
i assume you mean change "IBM & Microsoft" into "IBM & Microsoft", or even &.

if you know the codes of the chars you want to change, you can do something like the following:

my %ent=
(
  "&" => 38,
  "<" => 60,
  ">" => 62,
  #...
);

$string = "IBM & Microsoft > Sun\n";
print $string;

for ( keys %ent ) { $string =~ s/$_/&#$ent{$_};/g }

print $string;
#prints IBM &#38; Microsoft &#62; Sun
[download]

Another option is to put the string in question into a <![CDATA[...]]>-section.