When comparing a value against a constant, always write the constant to the left side like this:

if (undef = $WCkey) { ...
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If you make the error you made in your original post, Perl will immediately tell you.

Also, the function for checking whether a value is undef or not is the defined function.

Update: The if (undef = $WCkey) { ... is not a typo but intended for demonstration. Perl raises a runtime error when you use

if (undef = $WCkey) { ...
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instead of

if (undef == $WCkey) { ...
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while it doesn't even raise a warning if you write the "comparison" the other way around:

>perl -we "if (undef = $foo) { print 'bar' }"
Name "main::foo" used only once: possible typo at -e line 1.
Modification of a read-only value attempted at -e line 1.

>perl -we "if ($foo = undef) { print 'bar' }"
Name "main::foo" used only once: possible typo at -e line 1.
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So, in the spirit of building a defensive habit, write the constant to the left side of comparisons for equality.