After $utf8 = decode('foo', $octet); ,

  When $octet is...   The UTF8 flag in $utf8 is
  ---------------------------------------------
  In ASCII only (or EBCDIC only)            OFF
  In ISO-8859-1                              ON
  In any other Encoding                      ON
  ---------------------------------------------

##</code><code>##

#use utf8;
#use encoding 'iso-8859-1';
#use encoding 'utf-8-strict';
#use encoding::warnings;  #check for implicit upgrade
use Encode;

print '${^UNICODE}='.${^UNICODE}."\n\n";

$a = "face";
$b = "not_a_face=\x{e2}\x{98}\x{ba}";
print "a=",Encode::is_utf8($a)," b=",Encode::is_utf8($b),"\n\n";

$a=Encode::decode('iso-8859-1',$a);
$b=Encode::decode('iso-8859-1',$b);
print "a=",Encode::is_utf8($a)," b=",Encode::is_utf8($b),"\n\n";

##</code><code>##

${^UNICODE}=63
a= b=
a=1 b=1