Another option is to close the file, and then reopen it for reading:

use strict;
use warnings;

open my $fo, '+>>', "outfile.txt" or die "Error:$!\n";
print $fo "how r u\n";
print $fo "hey suresh their\n";
print $fo "in bangalore\n";
close $fo;

open my $fi, '<', "outfile.txt" or die "Error:$!\n";
while ( my $line = <$fi> ) {
    print "$line";
    my $a = index( $line, "suresh" );
    print "$a\n";
}

close $fi;
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Output:

how r u
-1
hey suresh their
4
in bangalore
-1
how r u
-1
hey suresh their
4
in bangalore
-1
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I'm assuming your meant index instead of substr.

thanks for the help

Now I am trying in different way, please correct if I am wrong

while ( <$fi> ) {
    #print "$_";
         
             my $a = $1 if /(^$name$)/;
             print $a;
  
}

close $fi;
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But when I run the file output show nothing, and it shows me some message, I am not getting where is the issue please see the below output

perl readwritefile.pl
Use of uninitialized value $a in print at readwritefile.pl line 15, <$
+fi> line 1.
Use of uninitialized value $a in print at readwritefile.pl line 15, <$
+fi> line 2.
Use of uninitialized value $a in print at readwritefile.pl line 15, <$
+fi> line 3.
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Hello ulaksh, Im guessing youve commented out your print "$_"; on purpose there. Anyway your getting yelled at because when the regex doesnt make a match, $a is not getting defined at all, so it is still undefined. You then try to use it on the next line print $a; You cant print $a because it has no value. It is undefined at the moment. This is also the reason your not getting anything printed to screen. maybe you can try this instead? :

print $1 if /^($name)$/;
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A much better general procedure is:
- open the input file
- make a new output file
- when you are finished, rename the orignal file to .bak or whatever and then rename the new file to the original name.

File system operations are complex and there is possibility when your program "bombs" or other errors happen and you will be left in an "unrecoverable situation" - the original data has been screwed up and there is maybe even no way, even with advanced techniques to recover the data.

Basically trying to modify an existing file is a very advanced and rarely used technique. I have software that does that but the problems and pitfalls are many - don't do it!