split the string(s) into a list of characters, use the map as shown, then join the result back together.

The method demonstrated below is devious and should be avoided in favour of something more conventional.

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

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

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

Update: Saner: (no nesting)

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

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

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