You don't tell us which operating system(s) you are using. If you're using a recent operating system like Windows NT, 2000 or XP, then there is Win32::ChangeNotify, which will allow you to conveniently monitor a directory or a whole tree. For other operating systems, like IRIX or (I believe) Linux, there is SGI::FAM, which also gives you notifications on file system changes.

I guess that NFS doesn't give you any notifications, so if you mount your drives via NFS, I guess you're out of luck.

So, if none of the options apply to your situation, you'll have to come forth with more details.

SGI::FAM

On linux, you can use the F_NOTIFY fcntl. See the fcntl(2) manpage.

#!/usr/bin/perl
use POE::Component::DirWatch;

# Take a look at the module POE::Component::DirWatch  
# on CPAN, I think this might be what you are looking  
# for. It watches a directory for files, and once a  
# filename matching the filter is found, it will kick  
# off a sub-process and invokes the given user callback. 

POE::Component::DirWatch->spawn(
   Alias        => 'dirwatch',
   Directory    => '/some_dir',
   Filter       => sub { $_[0] =~ /\.gz$/ && -f $_[1] },
   Callback     => \&some_sub,
   PollInterval => 1,
 );
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