Erm, undef without parens returns a perfectly valid value in that context. The problem is that it's being coerced into a numeric value because of the == which triggers the warning.

What you're thinking of is using undef or shift as a hash key ($h{undef}) or to the left of a => (%a = ( undef => 1 )) where there is some bareword magic going on (strict or no). In those cases it is necessary to use parens or a + to disable the magic.

Update: And you can verify it by looking at the different errors produced. Bareword-ness doesn't enter into it. 

freebie:~ 746> cat baz
use warnings;
if( 5 == undef ) { 1 }
if( 5 == "undef" ) { 1 }
freebie:~ 747> perl baz
Argument "undef" isn't numeric in numeric eq (==) at baz line 3.
Use of uninitialized value in numeric eq (==) at baz line 2.

[download]

The first one gripes about comparing against undef, the second because you're comparing a non-numeric value numerically.

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In reply to Re^2: Warning for checking with undef by Fletch
in thread Warning for checking with undef by icg

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