wheezy-x64$ cat foo.i
%module foo
%{
// Fill the supplied buffer with data:
void getBytes(unsigned char *buf, size_t len) {
    const unsigned char foo[] = {0xCA, 0xFE, 0xBA, 0xBE};
    size_t i = 0;
    while ( len-- ) {
        *buf++ = foo[i++];
        i &= 3;
    }
}
%}
%typemap(in, numinputs=0) unsigned char * {
    // The uint8* parameter is created by the binding and adopted by Perl, not
    // passed in by the user.
}
%typemap(check) size_t {
    retVal = newSV(arg2);     // allocate enough space
    sv_2mortal(retVal);       // make it mortal
    sv_setpv(retVal, "");     // make it defined (i.e not undef)
    SvCUR_set(retVal, arg2);  // set its length to its capacity
    arg1 = SvPVX(retVal);     // get a raw uint8* pointing to its storage
}
%typemap(out) void getBytes(SV *retVal = NULL) {
    ST(argvi) = retVal;
    argvi++;
}
void getBytes(unsigned char *, size_t);

wheezy-x64$ swig -perl5 foo.i
wheezy-x64$ gcc -m64 -g -fPIC -Dbool=char -I/usr/lib/perl/5.14.2/CORE -I. -DLINUX=2 -D_REENTRANT -D_LARGEFILE64_SOURCE -shared foo_wrap.c -o foo.so
wheezy-x64$ cat foo.pl
#!/usr/bin/perl -w
use foo;
$bytes = foo::getBytes(16);
print "Got ".length($bytes)." bytes:";
foreach ( unpack("(a1)*", $bytes) ) {
    print " ".sprintf("%02X", ord);
}
print "\n";
wheezy-x64$ ./foo.pl
Got 16 bytes: CA FE BA BE CA FE BA BE CA FE BA BE CA FE BA BE
wheezy-x64$