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