sub is_utf8 {
    utf8::upgrade( $_[0] );
    return 1;
}

[download]

But seriously, 

sub is_utf8 {
   my $s = "\x80" . $_[0];
   my $internal = unpack "p", pack "p", $s;
   return $s ne $internal;
}

[download]

Tested in 5.8.0, 5.8.8 and 5.10.0 using: 

utf8::downgrade( my $empty_dn = ''          );  # 0
utf8::upgrade(   my $empty_up = ''          );  # 1
utf8::downgrade( my $ascii_dn = 'a'         );  # 0
utf8::upgrade(   my $ascii_up = 'a'         );  # 1
utf8::downgrade( my $hibit_dn = chr(0xC9)   );  # 0
utf8::upgrade(   my $hibit_up = chr(0xC9)   );  # 1
utf8::upgrade(   my $wide_up  = chr(0x2660) );  # 1

for (
    $empty_dn, $empty_up,
    $ascii_dn, $ascii_up,
    $hibit_dn, $hibit_up,
    $wide_up,
) {
   print is_utf8($_)?1:0, "\n";
}

[download]

Why do you need to know?

Update: Added test code.

In reply to Re: good way to implement utf8::is_utf8 for perl 5.8.0 by ikegami
in thread good way to implement utf8::is_utf8 for perl 5.8.0 by perl5ever

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