I would not have thought that "( $an_undefined_variable == 0 )" would evaluate to true because, golly, undef isn't a number and so the "==" operator is testing whether one number is the same as another number.

The "==" operator is comparing the left hand side and the right hand side in numeric context.
And, in numeric context, undef and 0 are equivalent: 
C:\>perl -e "print 'ok' if undef == 0;"
ok
C:\>

[download]
But in string context, it's a different matter: 
C:\>perl -e "print 'ok' if undef eq 0;"

C:\>

[download]

Cheers,
Rob

In reply to Re: Surprising result when using undef by syphilis
in thread Surprising result when using undef by davebaker

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