Not that it makes any difference in this case, but defined returns  '' (the empty string; false) or  1 (true): 

c:\@Work\Perl>perl -wMstrict -le
"print 'perl version ', $];
 ;;
 print 'undefined A: >', defined(),      '<';
 print 'undefined B: >', defined(undef), '<';
 print 'defined: >',     defined(42),    '<';
"
perl version 5.008009
undefined A: ><
undefined B: ><
defined: >1<

perl version 5.014004
undefined A: ><
undefined B: ><
defined: >1<

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Give a man a fish:  <%-{-{-{-<

In reply to Re^2: "defined" function fails to evaluate expression; feature or fault? by AnomalousMonk
in thread "defined" function fails to evaluate expression; feature or fault? by w8lle

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