Hm.

#! 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;
};;

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 ); ],
};

__END__
C:\test>\perl5.18\perl\bin\perl.exe 1133857.pl
      Rate    B    A    C
B  43932/s   -- -48% -72%
A  84167/s  92%   -- -47%
C 159444/s 263%  89%   --
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