Sounds like you want transactions for your files, and rather conveniently, this is just what File::Transaction provides e.g

use File::Transaction;

my $ft = File::Transaction->new;

$ft->linewise_rewrite($fname);

## do stuff

if($file_contents_changed) { $ft->commit }
else                       { $ft->revert }
[download]

See. the File::Transaction docs for more info.
HTH

_________
broquaint