#! perl
use strict;
use warnings;

my $s = '< < &xyz >abc 123>456 me&you';
   $s =~ s/ (?<!&amp;) &lt; /</gx;
   $s =~ s/ (?<!&amp;) &gt; />/gx;
   $s =~ s/     &amp;lt;    /&lt;/gx;
   $s =~ s/     &amp;gt;    /&gt;/gx;
   $s =~ s/     &amp;       /&/gx;

print $s;