Another way to solve this is simply to initialize the variables before your split().

For example, I'm assuming your code looks something like this currently:

my ($this, $that, $other) = split (/\s+/, $line); 
# $line might have < 3 elements in it, the rest will be undef
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If you initialize your variables with non-undef's first, you'll avoid the "not defined" warnings:

my ($this, $that, $other) = ("") x 3;
($this, $that, $other) = split (/\s+/, $line); 
# Now the worst case is you'll have a null string, not undef
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Alan