>perl -wMstrict -le
"my $bad =
   'x &foo;xx;yz;qwe; y &de;fghj;h; z &>< y lt;gt; z';
 print qq{'$bad'};
 ;;
 my $tity = qr{ [[:alpha:]]+ ; }xms;
 ;;
 (my $fixed = $bad) =~
   s{ (?: (?: \G (?<! \A)) | &) $tity \K (?= $tity) }
    '&'xmsg;
 print qq{'$fixed'};
"
'x &foo;xx;yz;qwe; y &de;fghj;h; z &amp;&gt;&lt; y lt;gt; z'
'x &foo;&xx;&yz;&qwe; y &de;&fghj;&h; z &amp;&gt;&lt; y lt;gt; z'