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]

In reply to Re^3: add elements to array at certain positions by ikegami
in thread add elements to array at certain positions by coldy

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