Hi AnomalousMonk,

defined returns '' (the empty string; false) or 1 (true)

To be even more specific, Perl has a "special" value for "false" - see Truth and Falsehood.

The difference between an empty string and the special "false" value:

$ perl -wMstrict -le 'my $x=""; print 3+$x;'
Argument "" isn't numeric in addition (+) at -e line 1.
3
$ perl -wMstrict -le 'my $x=!!""; print 3+$x;'
3
$ perl -wMstrict -le 'my $x=defined(); print 3+$x;'
3

[download]

And looking even more closely with Devel::Peek:

$ perl -MDevel::Peek -e 'my $x=undef; Dump($x)'
SV = NULL(0x0) at 0x25ea5f8
  REFCNT = 1
  FLAGS = ()
$ perl -MDevel::Peek -e 'my $x=0; Dump($x)'
SV = IV(0x22ae518) at 0x22ae538
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 0
$ perl -MDevel::Peek -e 'my $x=""; Dump($x)'
SV = PV(0xa3e70) at 0xa390e8
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0xa398f0 ""\0
  CUR = 0
  LEN = 10
  COW_REFCNT = 1
$ perl -MDevel::Peek -e 'my $x=defined(); Dump($x)'
SV = PVNV(0x3f230e0) at 0x3fe6108
  REFCNT = 1
  FLAGS = (IOK,NOK,POK,pIOK,pNOK,pPOK)
  IV = 0
  NV = 0
  PV = 0x3f6a860 ""\0
  CUR = 0
  LEN = 10

[download]

Regards,
-- Hauke D

Update 2019-08-17: Updated the link to "Truth and Falsehood".

In reply to Re^3: "defined" function fails to evaluate expression; feature or fault? by haukex
in thread "defined" function fails to evaluate expression; feature or fault? by w8lle

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