Latest benchmark:

#!/usr/bin/env perl -slw

use strict;
use Benchmark qw[ cmpthese ];
use List::MoreUtils qw[ zip ];

sub zipA {
    my( $str1, $str2 ) = @_;
    $str1 =~ s/.\K/ substr $str2, 0, 1, ''/gesr;
}

sub zipB {
    no warnings qw/ uninitialized /;
    my( $a, $b ) = @_;
    my @a1 = split( '', $a );
    my @a2 = split( '', $b );
    return join'', zip @a1, @a2;
}

sub zipC($$){
   my( $n, $a, $b ) = ( 1, @_ );
   substr( $a, $n, 0, $_), $n += 2 for split '', $b;
   return $a;
};;

sub zipD {
    my ($str1, $str2) = @_;
    for (0 .. length $str2) {
        substr $str1, $_ * 2 + 1, 0, substr $str2, $_, 1;
    }
    return $str1;
}

our $A = 'ABCDEFGHIJ';
our $B = 'abcde';

cmpthese -1, {
    A => q[ my $zipped = zipA( $A, $B ); ],
    B => q[ my $zipped = zipB( $A, $B ); ],
    C => q[ my $zipped = zipC( $A, $B ); ],
    D => q[ my $zipped = zipD( $A, $B ); ],
};
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I ran it five times. This seems to be the most representative:

      Rate    B    A    C    D
B  91995/s   -- -39% -66% -72%
A 151837/s  65%   -- -45% -54%
C 274373/s 198%  81%   -- -17%
D 330831/s 260% 118%  21%   --
[download]