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);


##</code><code>##


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);