here wo go (by_asm is at the end) ==>
------------------------------------------------
use 5.8.0;

use strict;
use warnings FATAL => 'all';

use Benchmark qw( cmpthese );
use Test::More;

# ====================

my $s1 = do_rand(0, 100_000);
my $s2 = do_rand(1, 100_000);

my $subs = {
    split1    => sub { split1    ( my $s3 = $s1, $s2 );  $s3 },
    substr1   => sub { substr1   ( my $s3 = $s1, $s2 );  $s3 },
    moritz    => sub { moritz    ( my $s3 = $s1, $s2 );  $s3 },
    corion    => sub { corion    ( my $s3 = $s1, $s2 );  $s3 },
    ikegami1  => sub { ikegami1  ( my $s3 = $s1, $s2 );  $s3 },
    ikegami2  => sub { ikegami2  ( my $s3 = $s1, $s2 );  $s3 },
    bart      => sub { bart      ( my $s3 = $s1, $s2 );  $s3 },
    ikegami3  => sub { ikegami3  ( my $s3 = $s1, $s2 );  $s3 },
    ikegami4  => sub { ikegami4  ( my $s3 = $s1, $s2 );  $s3 },
    avar2     => sub { avar2     ( my $s3 = $s1, $s2 );  $s3 },
    avar2_pos => sub { avar2_pos ( my $s3 = $s1, $s2 );  $s3 },
    mrm_1     => sub { mrm_1     ( my $s3 = $s1, $s2 );  $s3 },
    mrm_3     => sub { mrm_3     ( my $s3 = $s1, $s2 );  $s3 },
    mrm_4     => sub { mrm_4     ( my $s3 = $s1, $s2 );  $s3 },
    mrm_5     => sub { mrm_5     ( my $s3 = $s1, $s2 );  $s3 },
    mrm_6     => sub { mrm_6     ( my $s3 = $s1, $s2 );  $s3 },
    by_asm    => sub { by_asm    ( my $s3 = $s1, $s2 );  $s3 },
};

{
    plan 'tests' => scalar keys %{$subs};

    my $expected;
    foreach my $subname ( keys %{$subs} ) {
        my $sub = $subs->{$subname};
        if ( defined $expected ) {
        is( $sub->(), $expected, "$subname gets same value" );
        }
        else {
        $expected = $sub->();
            ok( defined $expected, "$subname gets some value" );
        }
    }

    print("done.\n");
}

cmpthese( -3, $subs );

# ====================

sub split1 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    my @s1 = split //, $s1;
    my @s2 = split //, $s2;

    foreach my $idx ( 0 .. $#s1 ) { 
    if ( $s1[$idx] eq chr(0) ) { 
        $s1[$idx] = $s2[$idx];
    } 
    } 

    $s1 = join '', @s1;
}

sub substr1 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    for my $idx ( 0 .. length($s1) ) {
    if ( substr($s1, $idx,1) eq chr(0) ) {
        substr($s1, $idx, 1) = substr($s2, $idx, 1);
    }
    } 
} 

sub moritz {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    my $pos = 0;
    while ( -1 < ( $pos = index $s1, "\000", $pos ) ) {
    substr( $s1, $pos, 1 ) = substr( $s2, $pos, 1 );
    }
}

sub ikegami1 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    (my $mask = $s1) =~ s/[^\x00]/\xFF/g;
    $s1 = ($s1 & $mask) | ($s2 & ~$mask);
}

sub ikegami2 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    (my $mask = $s1) =~ tr/\x00/\xFF/c;
    $s1 = ($s1 & $mask) | ($s2 & ~$mask);
}

sub bart {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    (my $mask = $s1) =~ tr/\x00/\xFF/c;
    $s1 = (($s1 ^ $s2) & $mask) ^ $s2;
}

sub ikegami3 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    use bytes;
    (my $mask = $s1) =~ tr/\x00/\xFF/c;
    $s1 = (($s1 ^ $s2) & $mask) ^ $s2;
}

sub ikegami4 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    use bytes;
    (my $mask = $s1) =~ tr/\x00/\xFF/c;
    $s1 ^= $s2;
    $s1 &= $mask;
    $s1 ^= $s2;
}

sub corion {
   our $s1; local *s1 = \$_[0];
   our $s2; local *s2 = \$_[1];

   my $ofs = 0;
   $s1 = join "", map { $ofs += length; $_ => substr $s2, $ofs++, 1 } split /\0/, $s1, -1;
}

sub avar2 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    use bytes;
    $s1 =~ s/(\0)/substr $s2, $+[0]-1, 1/eg;
}

sub avar2_pos {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    use bytes;
    $s1 =~ s/\0/substr $s2, pos($s1), 1/eg;
}

sub mrm_1 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    # from [moritz]'s work
    use bytes;
    my $pos = 0;
    while ( -1 < ( $pos = index $s1, "\x00", $pos ) ) {
        substr( $s1, $pos, 1 ) = substr( $s2, $pos, 1 );
    }
}

sub mrm_3 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    # from moritz's, builds a separate loop of zeros
    use bytes;
    my @zeros = ();
    my $pos = 0;
    while ( -1 < ( $pos = index $s1, "\x00", $pos+1 ) ) {     # ikegam +i: Added necessary +1
        push @zeros, $pos;
    }
    for ( @zeros ) {
        substr( $s1, $_, 1 ) = substr( $s2, $_, 1 );
    }
}

sub mrm_4 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    # from [bart]'s vec()
   {
    use bytes;
    my $pos = 0;
    while ( -1 < ( $pos = index $s1, "\x00", $pos ) ) {
        vec( $s1, $pos, 8 ) ||= vec( $s2, $pos, 8 );
    }
   }
}

sub mrm_5 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    # from moritz's, seeing if four-arg substr() is faster or slower than lvalue substr()
    use bytes;
    my $pos = 0;
    while ( -1 < ( $pos = index $s1, "\x00", $pos ) ) {
        substr( $s1, $pos, 1, substr( $s2, $pos, 1 ) );
    }
}

sub mrm_6 {
    our $s1; local *s1 = \$_[0];
    our $s2; local *s2 = \$_[1];

    # from mrn_5, testing bytes::misc explicitly instead of importing
    use bytes ();
    my $pos = 0;
    while ( -1 < ( $pos = bytes::index( $s1, "\x00", $pos ) ) ) {
        bytes::substr( $s1, $pos, 1, bytes::substr( $s2, $pos, 1 ) );
    }
}

use Inline C => qq{
// ==> inline
 void by_asm(SV* has_zeros, SV* no_zeros)
{
 STRLEN spacer1, srclen, dstlen;
 char *src=SvPV(no_zeros, srclen), *spacer2=0, *dst=SvPV(has_zeros, dstlen);
 if( srclen < dstlen ) croak("block length mismatch!");
 
 #ifdef _MSC_VER
  _asm    mov   edi, dst
  _asm    mov   esi, src
  _asm    mov   ecx, dstlen
  _asm    xor   eax, eax
  _asm    cld
     start:
  _asm    repne scasb 
  _asm    jne   done
  _asm    mov   edx, dstlen
  _asm    sub   edx, ecx
  _asm    mov   ah, byte ptr [-1+esi+edx]
  _asm    mov   byte ptr [-1+edi], ah
  _asm    jmp   start
      done: ;
 #else
 __asm__ __volatile__( 
         "xorl  %%eax, %%eax          \\n\\t"
         "cld                         \\n\\t"
    "start:                           \\n\\t"
         "repne                       \\n\\t"
         "scasb                       \\n\\t"
         "jne   done                  \\n\\t"
         "movl  %[l], %%edx           \\n\\t"
         "subl  %%ecx, %%edx          \\n\\t"
         "movb  -1(%%esi,%%edx), %%ah \\n\\t"
         "movb  %%ah, -1(%%edi)       \\n\\t"
         "jmp   start                 \\n\\t"
    "done:                            \\n\\t"
     : /* no output reg */
     : "S"(src),"D"(dst),"c"(dstlen),[l]"m"(dstlen)
 );
 #endif
}
// <== inline
};

# ====================

sub do_rand {
    my $min = shift;
    my $len = shift;
    {
        my $n = "";
        for (1 .. $len)
        {
            $n .= chr( rand(255-$min)+$min );
        }
        redo if $min == 0 && $n !~ /\X00/;
        return $n;
    }
}

------------------------------------------------