Hello Everyone,
I figured out the solution to my problem . With some help from a tie example below, Ken was so kind to point out, and remembering the way arrays assign [0] [1] [2] and so on.
Well I am sure most of you knew this , But I will post it anyway I can not believe the fix for my original code was so simple , right under my nose. 

open(XFIL,"var.txt") || die $!;
        @main = <XFIL>;
                close(XFIL);



open(XFIL,">var.txt") || die $!;
 foreach $line (@main) {
  


@main[0]  =~ s/some data/this is the first line replaced/;  


print XFIL  $line ;    


}

close(XFIL);

[download]
 Hello again everyone, Sorry about being so cryptic last time I asked this. I will put my question in code. I created a way to do what I want but it seems so cumbersome. I am sure a better way exist? well here is the code

*Update This code replaces only ONE instance((meaning only one line) (well that is what the final result is)), not all. Hence one reason I created it in this bizarre fashion .
I am also new to programing ,another reason. Thanks again everyone for all your help 

open(XFIL,"var.txt") || die $!;
        @main = <XFIL>;
        close(XFIL);


open(XFIL,">var.txt") || die $!;
  foreach $line (@main) {
    $line =~ s/\n/replace/g; 
    print XFIL  $line;    
}
close(XFIL);



open(XFIL,"var.txt") || die $!;
   @main = <XFIL>;
close(XFIL);



open(XFIL,">var.txt") || die $!;
  foreach $line (@main) {
  
   $line =~ s/the line to be replaced/this is the new line/;  
   $line =~ s/replace/\n/g;
   print XFIL  $line;    
}

close(XFIL);

[download]

In reply to A Better Way by toniax

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