> Returns the number of characters actually read, 0 at end of file, or undef if there was an error

Therefore, the second version is equivalent to

$val = unpack 'H32', 16;
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because the read returned 16.

If you only specify the template, unpack uses the default variable $_ as the value to unpack.

You can use do to read and return the value you have just read:

$val = unpack 'H32', do { read FILNAM, $_, 16; $_ };
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But you should check what read returned to handle possible errors.

Also, calling a file handle FILNAM, which rather seems to be an abbreviation of "file name", is misleading.