If rogue chunk is e.g. 7 bytes long, then unpacking with the proposed template will die on "X", so wrapping into eval is required anyway if data are unreliable.

However, 'x' doesn't have a direct effect on the output, so a partial match involving 'x' must be communicated through an error.

Looks to me like an attempt to whitewash inconsistent Perl's behaviour :-), By similar reasoning, failure to unpack e.g. Pascal strings (as "unpack 'C/a', qq(\03ab)") should be fatal, I think.

Side-note: PNG tags were made human-readable for a good reason, so perhaps "A4" instead of "N" (or "L") will serve better. E.g., if data are super-reliable (CRC sums to be ignored), then chunks can be read into a hash:

my ( $head, %chunks ) = unpack 'a8 (x4 A4 X8 N x4 /a x4)*', $input;
say for keys %chunks;
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