'00FF' behaviour on z/OS

Hi,
If I run the following case folding test, 
$a = '0178';
$b = '00FF';

$a1 = pack("U0U*", hex $a);
$b1 = pack("U0U*", map { hex } split " ", $b);

if (":$b1:" =~ /:[$a1]:/i) {
print "ok\n";
}

it fails on z/OS (on perl-5.8.6).
However, if $b = '00DF', the test passes. 
Any thoughts on why this is happening ? Is there some behaviour specific to 
'00FF' that needs to be fixed ? 
 
Thanks in advance,
Rajarshi.

                
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