I tried it, but it did not change anything in the file

You didn't ask for help saving strings to a file, so I left that bit to you.

perl -pe"s/\bis\b/are/g" apache.txt >apache.new
[download]

Or here's a way that renames the original file for you and saves back to the original file:

perl -i.bak -pe"s/\bis\b/are/g" apache.txt
[download]

I'm afraid I do not understand your problem. If I take ikegami's code and modify it slightly so as not to depend on the existence of "apache.txt":

while (<DATA>) {
    s/\bis\b/are/g;
    print;
}

__DATA__
Apache is a modular server. This implies that only the most basic func
+tionality
is included in the core server. is it so ?
Extended features are available through modules which can be loaded in
+to Apache.
 By default, a base set of modules is
included in the server at compile-time. If the server is compiled to u
+se dynamic
ally loaded modules, then modules can be
compiled separately and added at any time using the LoadModule directi
+ve. Otherw
ise, Apache must be recompiled to add or
remove modules. Configuration directives may be included conditional o
+n a presen
ce of a particular module by enclosing
them in an <IfModule> block.
[download]

I get the following output (where clearly "is" has been replaced by "are" throughout):

Apache are a modular server. This implies that only the most basic fun
+ctionality

are included in the core server. are it so ?
Extended features are available through modules which can be loaded in
+to Apache.

 By default, a base set of modules are
included in the server at compile-time. If the server are compiled to 
+use dynami
c
ally loaded modules, then modules can be
compiled separately and added at any time using the LoadModule directi
+ve. Otherw

ise, Apache must be recompiled to add or
remove modules. Configuration directives may be included conditional o
+n a presen

ce of a particular module by enclosing
them in an <IfModule> block.
[download]

How does this differ from your desired output?

Apache is a modular server. This implies that only the most basic functionality is included in the core server. is it so ?

On this file , when I use following code

open(file, "apache.txt") ;
@modify = <file> ;

print @modify ;
foreach(@modify){
chomp($_);
$_ =~ s/\bis\b/are/g ;
# $_ =~ s/^is$/are/g ;
}
print @modify ;
[download]


This will replace 'is" with 'are' which I tested correcty. Now when I comment out "$_ =~ s/\bis\b/are/g ;" and uncomment "$_ =~ s/^is$/are/g ;" ,'is' will not be replaced by 'are' , which was my original problem. As ^ will look at the beginning of string and $ will look at the end of string.

If I add 'is' at the beginning and end of the string, will then 'is' be replaced with 'are' in the string ?

 is Apache is a modular server. This implies that only the most basic functionality is included in the core server. is it so ? is
If I use regex "$_ =~ s/^is$/are/g ;" on this text, will it replace 'is' with 'are' as this string has 'is' at the beginning and end as well, which satisfies the regex condition.
I tried, but it did not replace is with 'are'. So why did it not replace 'is' with 'are' ?

I tried it, but it did not change anything in the file

You didn't ask for help saving strings to a file, so I left that bit to you.

perl -pe"s/\bis\b/are/g" apache.txt >apache.new
[download]

Or here's a way that renames the original file for you and saves back to the original file:

perl -i.bak -pe"s/\bis\b/are/g" apache.txt
[download]

ikegami's script the the correct way to do it. It doesn't change anything in the file, rather it opens the file handle for output only. If you want to modify the file, that's a totally different procedure and isn't pertinent to the question that you asked about the regex.

Here's a way to modify a file in place:

First, make a copy of your file and put it on your desktop. In this example, I called it apache.txt.
Second, open a terminal.
Third, from the cmdline, enter:

perl -pi.orig -e 's/\bis\b/are/gi' /user/Desktop/apache.txt
[download]

It'll modify the original file and create a new apache.txt. It'll save the original as apache.txt.orig.