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This "False" value is basically the same as

use Scalar::Util qw/dualvar/; dualvar(0, '');

Which stems from the curious fact that a Perl scalar internally has a slot for both a numeric and a string value. By explicitly providing a numeric value, you don't get a warning when used as a number:

$ perl -wE 'say 1 + ""' Argument "" isn't numeric in addition (+) at -e line 1. 1 $ perl -MScalar::Util=dualvar -wE 'say dualvar(0, "") + 1' 1

Note that when abused, these dualvars can be very confusing:

$ perl -MScalar::Util=dualvar -E 'my $x = dualvar(42, "23"); say $x; s +ay $x + 1' 23 43

Devel::Peek shows that the False value is a wee bit more special than that:

perl -MDevel::Peek -MScalar::Util=dualvar -e 'Dump !1; Dump dualvar(0, + "")' SV = PVNV(0x254ff70) at 0x7409c0 REFCNT = 2147483647 FLAGS = (IOK,NOK,POK,READONLY,pIOK,pNOK,pPOK) IV = 0 NV = 0 PV = 0x254ff50 ""\0 CUR = 0 LEN = 8 SV = PVNV(0x2550d90) at 0x2551220 REFCNT = 1 FLAGS = (TEMP,IOK,POK,pIOK,pPOK) IV = 0 NV = 0 PV = 0x255f610 ""\0 CUR = 0 LEN = 8

So it's not only a dualvar, but it's also readonly, and has a very high refcount - it's basically a singleton.


In reply to Re^2: how ! operator works by moritz
in thread how ! operator works by nagalenoj

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