Are you putting your read lines into an array or a hash?

That was what I thought too ! To be sure I'm not leaking info..for all hashes, arrays and objects I do "undef var", after they are no longer needed.

Show us more of the code. Sounds like some restructuring is in order. In particular, anything that you "undef" ought to be inside the while loop so that it is cleaned up when it goes out of scope at the end of the loop. Only variables that should retain content after the loop should be declared outside the loop.

If you are not using strictures already I strongly recommend that you add use strict; use warnings; to your code.

---

DWIM is Perl's answer to Gödel

How long are the lines in the file? Unless they are extremely long what you have shown should be fine. Are you sure you are not leaking memory in the loop or accumulating data in the loop? As a sanity check you might like to try:

open my $fh,...
while (<$fh>) {
}
close $fh;
[download]

and check that that runs correctly. Assuming it does, start adding back the contents of the while loop and see where the problem happens. If it doesn't run correctly (terminate cleanly) you need to rethink what constitutes a line and possibly come back for more advice.

ooo - long lines or maybe a weird record separator

---

I humbly seek wisdom.

My guess is that you're doing something like

sub is_useful;

open $fh, '<', 'purr' or die "Sad kitty: $!";
my @useful_things = ();
while(<$fh>){
    push @useful_things if is_useful( $_ );
}
[download]

So all that data is ending up stashed in memory, even though you're reading the file a line at a time.