I tried three options. One using the pack('U0U*",...) solution from perluniintro, second using the regexps suggested by bart and a Encode::decode_utf8() using solution from also from peruniintro. The decode_utf8() is the fastest by far:

my $test = 0;

use warnings;
use bytes;
use Benchmark;
use Encode qw(encode_utf8 decode_utf8);

my $xml;

{
my $isUTF = 1;
my $sub = sub {$isUTF = 0};
sub byPack {
    $SIG{__WARN__} = $sub;
no warnings 'void';
    my @a=unpack( 'U0U*', $xml);
    delete $SIG{__WARN__};
    return $isUTF;
}
}

sub byRegExp {
    my $bad_utf8 = 0;
    while($xml =~ /(?=[\x80-\xFF])(?:[\xC0-\xFF][\x80-\xBF]+|(.))/g an
+d !$bad_utf8) {
        $bad_utf8++ if defined $1;
    }
    return !$bad_utf8;
}

sub byDecode {
    if (decode_utf8($xml)) {
        return 1
    } else {
        return 0
    }
}

print "OK\n";
open XML, '<test-ok.xml';
$xml = do {local $/; <XML>};
close XML;

if ($test) {
    print "byPack=".byPack()."\n";
    print "byRegExp=".byRegExp()."\n";
    print "byDecode=".byDecode()."\n";
} else {
    timethese (10000, {
        byPack => \&byPack,
        byRegExp => \&byRegExp,
        byDecode => \&byDecode,
    });
}

print "BAD\n";
open XML, '<test-bad.xml';
$xml = do {local $/; <XML>};
close XML;

if ($test) {
    print "byPack=".byPack()."\n";
    print "byRegExp=".byRegExp()."\n";
    print "byDecode=".byDecode()."\n";
} else {
    timethese (10000, {
        byPack => \&byPack,
        byRegExp => \&byRegExp,
        byDecode => \&byDecode,
    });
}
__END__
OK
Benchmark: timing 10000 iterations of byDecode, byPack, byRegExp...
  byDecode:  0 wallclock secs ( 0.22 usr +  0.00 sys =  0.22 CPU) @ 45
+662.10/s (n=10000)
            (warning: too few iterations for a reliable count)
    byPack: 15 wallclock secs (15.17 usr +  0.00 sys = 15.17 CPU) @ 65
+9.11/s (n=10000)
  byRegExp:  5 wallclock secs ( 4.22 usr +  0.00 sys =  4.22 CPU) @ 23
+70.79/s (n=10000)
BAD
Benchmark: timing 10000 iterations of byDecode, byPack, byRegExp...
  byDecode:  0 wallclock secs ( 0.08 usr +  0.00 sys =  0.08 CPU) @ 12
+8205.13/s (n=10000)
            (warning: too few iterations for a reliable count)
    byPack: 15 wallclock secs (15.42 usr +  0.00 sys = 15.42 CPU) @ 64
+8.42/s (n=10000)
  byRegExp:  5 wallclock secs ( 4.25 usr +  0.00 sys =  4.25 CPU) @ 23
+52.94/s (n=10000)
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The tests were run with two 4KB XMLs, the bad one had an í character added approximately in the middle.

Edit by castaway: Closed small tag in signature