'A' will remove trailing NULs and whitespace.
'a' will not.

In other words,
($str) = unpack('c/A', $data);
is equivalent to
($str) = unpack('c/a', $data);
$str =~ s/[\0\s]+$//;

In reply to Re^3: Length of unpacked string for "hex" data by ikegami
in thread Length of unpacked string for "hex" data by SirBones

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