Take a look at the use of the '/' in the documentation for pack, you can also use this format character in unpack.

Basically, an unpack format of 'C/a', will read the byte value represented by the C, and then use that as the length specifier for the character following the '/'; in this case 'A' for ascii data. As you also need the length of the metadata in order to remove it from the stream, you'll need a template of "a$DATASIZE C X C/A", which will capture the data to the first variable, the length to the second, backup over the length byte and then use it to capture the metadata to the third variable.

I've used a datasize of 10 and an array to simulate the read in this example. The critical part is exiting the while loop when there is not enough data left to fulfill the data size, then read the next lump and append it to the residual:

#! perl -slw
use strict;
use bytes;

my $DATASIZE = 10;
my @stream = (
    "abcdefghij\x04fredabcdefghij\x06barneyabcdefghij\x00abcde",
    "fghij\x07bam bamabcdefghij\x00abcdefghij"
);

my $stream = '';
for ( @stream ) {
    $stream .= $_;
    while( length( $stream ) > $DATASIZE) {
        my( $data, $len, $meta ) = unpack "a$DATASIZE C X C/A", $strea
+m;
        print "\ndata:$data";
        print "meta:$meta" if $len;
        my $trim = $len ? $len+1 : 1;
        $stream = bytes::substr( $stream, $DATASIZE + $trim );
    }
}
__END__
c:\test>junk

data:abcdefghij
meta:fred

data:abcdefghij
meta:barney

data:abcdefghij

data:abcdefghij
meta:bam bam

data:abcdefghij
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