Side-effect of Perl's print function

Question

I'm wondering why/how print in Perl can have a side-effect.

use Scalar::Util qw/looks_like_number/; 

my @A = (5, '2', 'aaa', 1, 'aab'); 

my @a = map { looks_like_number($_) } @A; 
print "1) @a\n";
# prints "4352 1 0 4352 0"

print "A print with a side-effect: @A\n";

@a = map { looks_like_number($_) } @A; 
print "2) @a\n";
# prints "1 1 0 1 0"

In this example, looks_like_number returns 4352 1 0 4352 0 before the print, and 1 1 0 1 0 after the print.

What does print do to these values to affect how they're interpreted by looks_like_number?

Answer 1

When interpolating or otherwise concatenating a numeric value, a stringified version needs to be created. This string is stored within the scalar (in addition to the numeric value) for later use, and this can affect which value looks_like_number returns.

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To inspect variables' internals, use Devel::Peek^[1].

use Devel::Peek qw( Dump );
my @A = (5, '2', 'aaa');
Dump($_) for @A;  # Or: Dump(\@A);
print "@A\n";
Dump($_) for @A;

With Perl 5.20, you get the following: (Rerranged for readability)

Before                             After
===============================    ===============================
SV = IV(0x4532a78) at 0x4532a88    SV = PVIV(0x45563a0) at 0x4532a88
  REFCNT = 2                         REFCNT = 2
  FLAGS = (IOK,pIOK)                 FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 5                             IV = 5
                                     PV = 0x454a870 "5"\0
                                     CUR = 1
                                     LEN = 10

SV = PV(0x45336a0) at 0x4532c08    SV = PV(0x45336a0) at 0x4532c08
  REFCNT = 2                         REFCNT = 2
  FLAGS = (POK,IsCOW,pPOK)           FLAGS = (POK,IsCOW,pPOK)
  PV = 0x455cf00 "2"\0               PV = 0x455cf00 "2"\0
  CUR = 1                            CUR = 1
  LEN = 10                           LEN = 10
  COW_REFCNT = 1                     COW_REFCNT = 1

SV = PV(0x4533720) at 0x4550b90    SV = PV(0x4533720) at 0x4550b90
  REFCNT = 2                         REFCNT = 2
  FLAGS = (POK,IsCOW,pPOK)           FLAGS = (POK,IsCOW,pPOK)
  PV = 0x455f210 "aaa"\0             PV = 0x455f210 "aaa"\0
  CUR = 3                            CUR = 3
  LEN = 10                           LEN = 10
  COW_REFCNT = 1                     COW_REFCNT = 1

Look at the FLAGS. When provided a number, the stringification of the number is cached (stored in the scalar) for future use. This is the side-effect mentioned.

The value returned by looks_like_number is sometimes a subset of the bits of FLAGS^[2]. This is why the specific value returned varies. It will still return true if it returned true before, and it will still return false if it returned false before.

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1. illguts provides information on the format of scalars.

2. See Behavior of Scalar::Util's looks_like_number.