if ($STRING =~ m/[^\x00-\xFF]{1}/)
{
  ...
}

##</code><code>##

if (length($STRING) > ($STRING =~ tr|\x00-\xFF|\x00-\xFF|))
{
  ...
}

##</code><code>##

sub isUnicode
{
  my $L = defined $_[0] ? length($_[0]) : 0;
  for (my $i = 0; $i < $L; $i++)
  {
    ord(substr($_[0], $i, 1)) < 256 or return 1;
  }
  return 0;
}