A little variation of your code (with a null byte inside the stream that is read by the first Z pattern): 

use v5.10;
my $str = qq{abc\0def};
my $pck = pack q{Z*}, $str;
say sprintf q{%02x}, ord for split m{}, $pck;
say q{-} x 5;
say join ",", unpack q{Z5Z*}, $pck;
__DATA__
61
62
63
00
64
65
66
00
-----
abc,ef

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The way I see it is that the Z5 pattern makes perl read the first five bytes and strip away everything after the first null byte (ie: the last byte with value 64 is stripped) to obtain the bytes (61, 62, 63, 00). Those four bytes are not yet a perl string but "A null-terminated (ASCIZ) string", where the null byte is part of the formatting, not the data. It is the translation from one format (null-terminated string) to another (perl string) that makes the null byte disappear, not the stripping.

With that interpretation it's the fact that you can do unpack "Z4", "hello", where the obtained data is not a valid null-terminated string, that might be surprising, but that's just perl Doing What You Mean as it usually does.

In reply to Re: Behaviour of unpack() with the Z template by Eily
in thread Behaviour of unpack() with the Z template by johngg

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