Because undef *a undefines the typeglob *a, but not the underlying slots. Look at these 2 examples:

#1
$a = 10;
*b = *a;
$a = 4;
print $b; # <- prints 4

#2
$a = 10;
*b = *a;
undef *a;
$a = 4;
print $b; # <- prints 10!
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