Your code does not compile.

Please post code that actually compiles instead of typing it in separately.

Also, you may want to think about what the following line is supposed to do:

foreach(my $line =  <$FH>) {
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... maybe you want to learn about what readline resp. the diamond operator does in scalar context. Most likely, while is a better approach.

i tried while loop also but it doesn't showing what I'm expecting

It seems you forgot a lot of what you were shown in how read data from file. Since you ignored my other advice you'll likely ignore this pointer also.

$/ to read whole record and foreach is to read every line

$/ tells readline or <> to read the whole file. foreach does not read anything, it just iterates a list. You give it a one member list: it contains the whole file as its only member.
لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

Why don't you just read the file line-by-line?

I did some changes in to your code & was able to achieve what you were looking.Here is the code

#!/usr/bin/perl 
use strict;
use warnings;

use strict;
use warnings;
use diagnostics;
use Modern::Perl;
open FH, '<', 'Perl_file.txt' or die "Could not open file= $!";

while( <FH>) {                        
    chomp($_);                    
    my $line =$_;
    $line =~ s/\s+//gs;
    my $char_a = substr($line, 0, 2);
    my $char_b = substr($line,2,2);
    print $char_a;
    print $char_b;
                               
}
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Data i inserted in to Perl_file.txt was

Raam
larry
marry
sephali
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OUTPUT:

You rightly removed the line returns with chomp, you need to add them back when you print:

print $char_a, "\n";
print $char_b, "\n";
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Additional comments: why do you have use strict and use warnings twice (actually three times since you are also using modern Perl)? Why did you change from a lexical filehandle ($FH) to a bare word filehandler (FH)?