It is not clear for me what you want to accomplish, but generally use HTML::Entities to process html entities:
my $line="” 3&rdquo;";
If this module is of no help, please, try to give a step backwards and explain the kind of conversions you want to be done
Update: A closer look at your code reveals that you are not using alternation correctly:  in a regexp is a character class, but probably you are wanting () instead (see perlretut and perlre):
$line="” 3 &rdquo;";
$line =~ s/(?:&|&\;)rdquo;/'/g;
' 3 '
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