Yuck! Using unpack is inappropriate here! Use substr:

push(@{$a[$_%2]}, substr($b, $_, 1))
   for 0..13;
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Alternatively:

push(@{$a[0]}, $b =~ /(.)./g);
push(@{$a[1]}, $b =~ /.(.)/g);
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And now for something fun (but not recommended):

my $t = 1;
push(@{$a[$t ^= 1]}, $_)
   foreach $b =~ /(.)/g;
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By the way, you shouldn't name your variables $b (or $a). These are reserved for sort.