I need a dynamic number of bytes incoming, without having to change the underlying C code,

I'm not quite sure why you think you cannot pass variable length strings in scalars?

Also, whilst pack is one way to encode unsigned bytes into strings, it is not the only way:

#! perl -slw
use strict;
use Inline C => Config => BUILD_NOISY => 1;
use Inline C => <<'END_C',  NAME => '_1180095', CLEAN_AFTER_BUILD =>0;

int doStuff( SV *sv ) {
    STRLEN len, i;
    unsigned char *bytes = SvPVx( sv, len );

    printf( "%d : ", len );
    // print the hi & lo nybbles, of the unsigned bytes passed, in hex
    for( i=0; i < len; ++i ) {
        printf( "%01x %01x\t", ( bytes[ i ] & 0xf0 ) >> 4, bytes[ i ] 
+& 0x0f );
    }
    printf( "\n" );
    return 1;
}

END_C


doStuff( "\x0f\xf0\xaa\x55\x01\x80" );

doStuff( join '', map chr, 0x00 .. 0xff );

doStuff( pack 'C*', 1 .. 10 );
[download]

Output:

C:\test>1180095.pl
6 : 0 f    f 0     a a     5 5     0 1     8 0

256 : 0 0       0 1     0 2     0 3     0 4     0 5     0 6     0 7   
+  0 8     0 9     0 a     0 b     0 c     0 d     0 e     0 f     1 0
+     1 1     1 2     1 3     1 4     1 5     1 6     1 7     1 8     
+1 9     1 a     1 b     1 c     1 d     1 e     1 f     2 0     2 1  
+   2 2     2 3     2 4     2 5     2 6     2 7     2 8     2 9     2 
+a     2 b     2 c     2 d     2 e     2 f     3 0     3 1     3 2    
+ 3 3     3 4     3 5     3 6     3 7     3 8     3 9     3 a     3 b 
+    3 c     3 d     3 e     3 f     4 0     4 1     4 2     4 3     4
+ 4     4 5     4 6     4 7     4 8     4 9     4 a     4 b     4 c   
+  4 d     4 e     4 f     5 0     5 1     5 2     5 3     5 4     5 5
+     5 6     5 7     5 8     5 9     5 a     5 b     5 c     5 d     
+5 e     5 f     6 0     6 1     6 2     6 3     6 4     6 5     6 6  
+   6 7     6 8     6 9     6 a     6 b     6 c     6 d     6 e     6 
+f     7 0     7 1     7 2     7 3     7 4     7 5     7 6     7 7    
+ 7 8     7 9     7 a     7 b     7 c      7 d     7 e     7 f     8 0
+     8 1     8 2     8 3     8 4     8 5     8 6     8 7     8 8     
+8 9     8 a     8 b     8 c     8 d     8 e     8 f     9 0     9 1  
+   9 2     9 3     9 4     9 5     9 6     9 7     9 8     9 9     9 
+a     9 b     9 c     9 d     9 e     9 f     a 0     a 1     a 2    
+ a 3     a 4     a 5     a 6     a 7     a 8     a 9     a a     a b 
+    a c     a d     a e     a f     b 0     b 1     b 2     b 3     b
+ 4     b 5     b 6     b 7     b 8     b 9     b a     b b     b c   
+  b d     b e     b f     c 0     c 1     c 2     c 3     c 4     c 5
+     c 6     c 7     c 8     c 9     c a     c b     c c     c d     
+c e     c f     d 0     d 1     d 2     d 3     d 4     d 5     d 6  
+   d 7     d 8     d 9     d a     d b     d c     d d     d e     d 
+f     e 0     e 1     e 2     e 3     e 4     e 5     e 6     e 7    
+ e 8     e 9     e a     e b     e c     e d     e e     e f     f 0 
+    f 1     f 2     f 3     f 4     f 5     f 6     f 7     f 8     f
+ 9      f a     f b     f c     f d     f e     f f

10 : 0 1        0 2     0 3     0 4     0 5     0 6     0 7     0 8   
+  0 9     0 a

C:\test>
[download]