c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = '&  <  >  <  >  &<  &&';
 print qq{'$s'};
 ;;
 my %replace = ( amp => '&',  lt  => '<',  gt  => '>', );
 my ($find) =
   map qr{ & ($_) ; }xms,
   join '|',
   keys %replace
   ;
 $s =~ s{$find}{$replace{$1}}xmsg;
 print qq{'$s'};
"
'&amp;  &amp;lt;  &amp;gt;  &lt;  &gt;  &amp;&lt;  &amp;&amp;'
'&  &lt;  &gt;  <  >  &<  &&'

##</code><code>##

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = '&amp;  &amp;lt;  &amp;gt;  &lt;  &gt;  &amp;&lt;  &amp;&amp;';
 print qq{'$s'};
 ;;
 my %replace = ( amp => '&',  lt  => '<',  gt  => '>', );
 my ($entity) =
   map qr{ $_ }xms,
   join ' | ',
   keys %replace
   ;
 print $entity;
 ;;
 $s =~ s{ & ($entity) ; }{$replace{$1}}xmsg;
 print qq{'$s'};
"
'&amp;  &amp;lt;  &amp;gt;  &lt;  &gt;  &amp;&lt;  &amp;&amp;'
(?^msx: lt | gt | amp )
'&  &lt;  &gt;  <  >  &<  &&'