One way to verify what Perl is really storing internally is Devel::Peek. What I would look for is that the UTF8 flag is on, and the string when shown as UTF-8 is correct:

use Devel::Peek;
my $str = "\x{20AC}";
Dump($str);

__END__

SV = PV(0x15c0d70) at 0x15e0440
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK,UTF8)
  PV = 0x15e4e10 "\342\202\254"\0 [UTF8 "\x{20ac}"]
  CUR = 3
  LEN = 10
  COW_REFCNT = 1
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Here, [UTF8 "\x{20ac}"] is correct. There's also utf8::is_utf8($str) to check for the UTF8 flag, although I'd recommend only using that for debugging as well. If you don't want all the extra output, you might just say:

use Data::Dump;
my $str = "\x{20AC}";
dd $str, utf8::is_utf8($str);
__END__
("\x{20AC}", 1)
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