Yet another WTDI:

my ($string_a, $string_b) = ('ABABA','BB--B-BB');
my @a = split '',$string_a;
my @b = split '',$string_b;
$string_a = '';

my $a_ndx = 0;
for (0..@b-1) { $string_a .= $b[$_] eq '-' ? '-' : $a[$a_ndx++] }
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This particular version modifies the 'A' string, but it could be trivially modified to preserve that string and build a fresh one; I consider that an advantage, but YMMV.