The silent and sporadic errors are the nasty ones (from my PoV). Some ideas:

• Control flow: A conditional stray last or next in a subroutine that is called from within a loop can have interesting effects.
• Variable names: Take a descriptive name but use it for something else, e.g. store user-ids in %IP_Tables. Or use something like $result_aref but don't let it refer to an array.
• I/O: print in combination with select offers some pitfalls...
• List context: remove or displace some parenthesis ...
• Return values: implicit return values (instead of a proper return)...
• Side effects (global variables w. and w/o local)...
• Implicit use of @_, e.g. by &some_sub; (see perlsub)...
• shift can operate on @_ or @ARGV...
• poison input: remove/add some TAB characters... swap some columns...
• ...

Is this approach intended to test a maintenance programmer? Maybe I got something wrong, but I feel a little bit uncomfortable with the approach of laying out a code-minefield. Wouldn't it be more constructive to give the candidate a task to start from scratch? ... or to find an alternative solution to the already solved problem? ... or let her/him optimise/refactor the script concerning speed/memory/code-formatting/robustness? ... turn a script into a module? ... wrap a GUI around it? ...

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