Just noting that with the exception of Merlyn's solution none of the answers given here implement any kind of authentication of the sender before running the script, which means that they have a security hole which allows any-one to trigger execution the script- checking mail headers will not work as these can be spoofed.

While it may be unlikely that someone will take the time to compromise your script or the harm minimal if they did it would be well worth the effort to at least understand Merlyn's solution. You can't spend too much time considering security.

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my $chainsaw = 'Perl';