From the question, it really sounds to me that the file is already open, and that the OP doesn't want to close it and re-open it. In which case, they are looking for truncate.

open my $fh, ">", "./foo.txt" 
   or die( "Couldn't etc." );
print $fh "$_\n" for 1..100;

truncate $fh, 0;

print $fh "$_\n" for 1..10;
close $fh;
[download]

Now foo.txt contains 1..10 only.

Update: As parv mentioned up-thread, this might close and re-open under the hood. It's not clear if you don't want to re-open for programatic reasons or for performance reasons.