Inline::C?

Update: Bleh this won't work with nulls because it's returning a vanilla C char*. You'd need to take and return an SV* instead, but you get the idea.

$ perl interleave.plx
0.0027470588684082
0.410820007324219

#!/usr/bin/env perl
use strict;
use warnings;

use Inline 'C';
use Time::HiRes qw( time );

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

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

__END__
__C__
char *interleave(char *in, char other) {
    char *outbuf = malloc( sizeof(char) * strlen( in ) * 2 + 1);

    char *outp = outbuf;
    char *p = in;

    while( *p ) {
        *outp++ = *p++;
        *outp++ = other;
    }

    *outp = '\0';

    return outbuf;
}
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