my $file = "c:/test.txt";

# open file for both reading and writing
open FILE, "+<$file" or die "Can't open $file: $!\n";
my @file = <FILE>;         # get all data
make_backup($file, @file); # make a backup
seek FILE, 0, 0;           # go to beginging of file
truncate FILE, 0;          # delete all the old data

# now write only the lines we want back to file
# the newlines are still attached to each line
for my $line (@file) {
    print FILE $line unless $line eq "don't print\n";
}
close FILE;

# sub to write a backup
sub make_backup {
    my $file = shift;
    open BACKUP, ">$file.bak" or die "Can't write $file.bak: $!\n";
    print BACKUP @_;
    close BACKUP;
}