open(F, '>:utf8', "new.txt"); 
print F[7] "$myvar"; 
close F; 
 
something like this, 
or am i going the wrong way about this

open(F, '>:utf8', "new.txt");

print F[7] "$myvar";

chomp $line; 
if ($. == 11) { 
     # change line 11 somehow... 
}

my $file = "1.txt"; 
my $filenew = "2.txt"; 
my $myvar = "NEW STRING"; 
 
open my $in,  '<',  $file      or die "Can't read old file: $!"; 
open my $out, '>', "$filenew" or die "Can't write new file: $!"; 
 
 
while( <$in> )   # print the lines before the change 
    { 
     
    last if $. == 4; # line number before change 
print $out $_; 
    } 
 
my $line = "$myvar\n"; 
 
print $out $line; 
 
while( <$in> )   # print the rest of the lines 
    { 
    print $out $_; 
    } 
close $in; 
close $out; 
unlink( "1.txt" ); 
rename("2.txt", "1.txt")

my $line_11 = "I have a dream, that my four little children will one day live in a nation where they will not be judged by the color of their skin ...\n"; 
while ($line = <FILE>) { 
     if ($. == 11) { 
          print OUT $line_11; 
     else 
          print OUT $line; 
     } 
}

my $newstring = "this is my new line to replace the old 1\n";  
 
my $infile = "1.txt";  
my $outfile = "2.txt";  
open FILE, "<", $infile or "die cannot open $infile $! \n";  
open OUT, ">", $outfile or "die cannot open $outfile $! \n";  
while (my $line = <FILE>) {  
     if ($. == 11) {  
          print OUT $newstring; } 
     else { 
          print OUT $line;  
     }  
} 
close FILE;  
close OUT;  
 
unlink('1.txt'); 
rename('2.txt','1.txt');