Here you go: 
#!/usr/bin/env perl

use strict;
use warnings;

use Inline C => <<'__EOI__';

/* sv_in and sv_pad may not be NULL        */
/* That won't happen when called from Perl */
SV* interleave_bytes(SV* sv_in, SV* sv_pad) {
    STRLEN l_in;   char* p_in  = SvPVbyte(sv_in,  l_in);
    STRLEN l_pad;  char* p_pad = SvPVbyte(sv_pad, l_pad);
    char   pad;
    SV*    sv_out;
    char*  p_out;

    if (l_pad != 1)
        croak("usage");
    pad = *p_pad;

    sv_out = newSVpvn("", 0);
    p_out = SvGROW(sv_out, l_in*2+1);  /* XXX Could overflow */
    SvCUR_set(sv_out, l_in*2);
    while (l_in--) {
        *(p_out++) = *(p_in++);
        *(p_out++) = pad;
    }
    *p_out = '\0';
    return sv_out;
}

__EOI__

use Time::HiRes qw( time );

my $buf = chr(1) x 1e6;;
my $s = time; 
my $out = interleave_bytes( $buf, chr(0) );
print time() - $s, "\n";

#use Devel::Peek;
#Dump $out;

$buf = chr(1) x 1e6;;
$s = time; 
$out = join( chr(0), unpack '(A1)*', $buf ) .chr(0); 
print time() - $s, "\n";

[download]
0.00459790229797363
0.446739912033081

[download]

Update: Fixed bugs mentioned elsewhere in thread.

In reply to Re^2: Interleaving bytes in a string quickly by ikegami
in thread Interleaving bytes in a string quickly by BrowserUk

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