Assuming that your undefined values are coming in from subroutine calls, or from something like CGI.pm, one way I do it is to set a default value for it using the powerful ||= operator: 
sub do_something {
   my ( $file ) = @_;
   $file ||= "default.txt";
   ...
}

[download]
If $file is defined, no further action is taken, since || short-circuits out on the true value. Otherwise, it sets $file to the default name given. Any time further, there's very little chance that $file will be undef'd.

The only time this doesn't work is if a value 0 or the empty string ('') is passed, since these also evaluate to false. In cases where these are possible values, I then do defer to defined to make sure that the values aren't overwritten.

-----------------------------------------------------
Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
It's not what you know, but knowing how to find it if you don't know that's important

In reply to Re: on to better coding (no uninitialized values) by Masem
in thread on to better coding (no uninitialized values) by melguin

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