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use List::MoreUtils qw(indexes);
my $a = '0157953';
...
substr $new, $_, 0, 'I' foreach ( indexes { $_ =~ /^[yz]$/ }
                                  split(//, $b) );
print $new, "\n";

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