Oops, I'm sorry! I will stop posting at night before sleep. Here is a fast List::MoreUtils alternative that actually does what the OP asks for this time:

use List::MoreUtils qw(indexes);
my $a = '0157953';
my $b = 'abcyefzaa';
my $new = $a;
substr $new, $_, 0, 'I' foreach ( indexes { $_ =~ /^[yz]$/ }
                                  split(//, $b) );
print $new, "\n";
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Update: I just realized this nearly identical loop is much faster (nested update: oops it's pos-1):

substr $new, pos($b)-1, 0, 'I' while ( $b =~ /[yz]/g );
[download]