use strict;
use warnings;

use IO::InSitu;

my $infile_name   = 'fred.tmp';
my $outfile_name  = $infile_name;

my ($in, $out) = open_rw($infile_name, $outfile_name);

for my $line (<$in>) {
    print {$out} transform($line);
}

# Try pressing CTRL-C while file is being updated.
sub transform {
    sleep 1;
    return "hello:" . $_[0];
}

##</code><code>##

total 20
drwxrwxr-x   2 andrew andrew 4096 Sep  3 14:44 ./
-rw-rw-r--   1 andrew andrew    0 Sep  3 14:42 fred.tmp
-rw-rw-r--   1 andrew andrew  191 Sep  3 14:42 fred.tmp.bak
drwxrwxr-x  11 andrew andrew 4096 Sep  3 14:42 ../
-rw-rw-r--   1 andrew andrew  288 Sep  3 14:41 tsitu1.pl