open my $ZIP, '+<', 'test.zip';
local $/ = undef;
my $data = <$ZIP>;
my @offset;
while ($data =~ /PK\x03\x04/g) { # find all file headers
   push @offset, pos($data) - 4;
}
print "Found headers at: @offset\n";
substr($data, $offset[1]+14, 1) ^= "\x01"; # change the crc
seek $ZIP, 0, 0;
print $ZIP $data;