I want to catch the warning "Unicode non-character U+xxxx is illegal for open interchange", so I can output something else (that's valid).

The best approach I've been able to devise is to output to a string inside an eval, as follows. That works ok (as far as I can see), but seems excessively complex. I seek wisdom as to whether there is a simpler way. Here is the code I have:

my $arg = "10FFFF"; # for instance

# simpler way?
eval { use warnings FATAL => qw(all);
  my ($fh, $string);
  open($fh, ">", \$string) || die "open(string) failed: $!";
  binmode($fh, ":utf8");
  print $fh chr(hex("$arg"));
};

die "good, caught it from eval: $@" if $@;

# don't want to get here
binmode(STDOUT, ":utf8");
print chr(hex($arg));
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I've attempted to grok tchrist's exhaustive answer at stackexchange. As near as I can make out from that, the warning explicitly and correctly only happens on output.

Looking at the source (utf8.c), I lack the brainpower to comprehend whether the Perl_uvoffuni_to_utf8_flags() function which emits the warning is called under other circumstances.

Blindly looking at that code, it seems that what I'm imagining is Perl-level access to the UNICODE_IS_NONCHAR() macro, but it seems that that, as such, doesn't exist. Looking at its definition in utf8.h, I certainly don't want to reimplement it, even if I could.

Any ideas welcome. Thanks.