Whereas testing defined($var) is arguably better style; I feel I should point out that the use of undef shown in the responses above is wrong for the wrong reason with respect to the OP.

undef needs to be parsed as a function call in this context, so when used in the context of a relational expression, it needs round brackets after it to prevent it from being treated as a bareword. i.e. 

if ( $foo == undef() )

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would work just as well as 

unless( defined($foo) )

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whereas

if ( $foo == undef )

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fails to satisfy for the same reason as

if ( $foo == DoesntMatterIfIamADefinedFunctionOrNot )

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The same programming error is often made with shift();

One world, one people

In reply to Re: Warning for checking with undef by anonymized user 468275
in thread Warning for checking with undef by icg

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