replace string

sandy1028 has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I have to read the file and if I find the characters like &ldquo,&rdquo,&rsquo I have to replace it with some characters.

$data =~ s/&ldquo;/&#8220;/g; # change the lines
$data =~ s/&rdquo;/&#8221;/gi;
$data =~ s/&rsquo;/&#8217;/gi;
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But this is not replaced in the file when it occurs. How to do this