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

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

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