In a recent thread I was sufficiently convinced by other monks that Tie::File - even though I still think it's kind of "neat" - is not a good choice in a serious application. On a large file it's much too inefficient, and even on a small file it's still more efficient to just read the entire file into an array and write it back out after modifying. So either:

open my $ifh, '<', $filename or die "$filename: $!";
chomp( my @array = <$ifh> );
close $ifh;

# modify @array here

open my $ofh, '>', $filename or die "$filename: $!";
print $ofh $_,"\n" for @array;
close $ofh;
[download]

Or, to demonstrate a while(<>) loop and plug one of my own modules ^;-) see File::Replace:

use File::Replace;
 
my $repl = File::Replace->new($filename);
my $infh = $repl->in_fh;
while (my $line = <$infh>) {
    chomp($line);
    # modify $line here
    print {$repl->out_fh} $line, "\n";
}
$repl->finish;
[download]

As for the problem you're having with Tie::File, I think both tangent and toolic have a very good point about write permissions. I would also add that perhaps using proper modules to handle file names platform independently might help with mitigating headaches about slashes etc. in filenames: either the core File::Spec, or, with a much nicer interface, Path::Class (there's also Path::Tiny with a similar interface, but be aware that that's restricted to Windows an *NIX).