roughly 5-7 times faster!

$mult=1e7;
 
$buf = chr(1) x $mult;
$s = time; 
$out = join( chr(0), unpack '(A1)*', $buf ) .chr(0); 
print "TIME0:",time() - $s,"\n";
 
 
 
$del=pack('(B16)*','00000000'.'11111111')x $mult;
$buf="ABCDEFGHIJ"x($mult/10);
 
$s = time(); 
#$buf2=pack('(B16)*',unpack('(B8)*',$buf));
$buf2=pack 'v*', unpack 'C*', $buf;            # jmcnamaras pack-unpac
+k-ing is much faster !!!
$out = $del|$buf2;
print "TIME1:",time() - $s,"\n";
 
print join ",",unpack '(B8)20',$out;           # check result
[download]

OUT:

TIME0:14
TIME1:2
01000001,11111111,01000010,11111111,01000011,11111111,01000100,1111111
+1,01000101,11111111,01000110,11111111,01000111,11111111,01001000,1111
+1111,01001001,11111111,01001010,11111111
[download]

UPDATE: even if it's a one-time task you can profit from this approach by chunking the string into eqally sized pieces which are "or"ed against the precalculated $del string.

UPDATE: even if you include calculating $del it's about 3 times faster!