use Encode qw( is_utf8 );

my $s1 = "abc";
my $s2 = chr(0x2660);
my $s = $s1 . $s2;
print("s1: ", is_utf8($s1)?1:0, "\n");  # 0
print("s2: ", is_utf8($s2)?1:0, "\n");  # 1
print("s:  ", is_utf8($s )?1:0, "\n");  # 1