my $stra = '0157953';
my $strb = 'abcyefzaa';

my $new = '';
while ($strb =~ /([yz])|./sg) {
   $new .= (defined($1)
      ? 'I'
      : ($stra =~ /(.)/sgc ? $1 : die )
   );
}

print("$new\n");  # 015I79I53