## UNTESTED
use File::Temp 'tempfile';
use File::Copy;
my ($tmp_FH,$tmp_fn) = tempfile();

open(IN, '<', $ifile) or die "Couldn't open file: $!\n";
my @data = <IN>;
close IN;

foreach my $line (@data) {
    ### Do what you want
    print $tmp_FH "$line";
}
copy($tmp_fn,$ifile);