Just to be painfully clear,  for(each) loops (and map and grep also) implicitly localize the  $_ default scalar;  while loops do not.

Consider the behavior when  $_ is not protected from the side-effects of the  while loop:

>perl -wMstrict -MData::Dumper -le
"my @array = qw(A B C);
 $_ = 'foo';
 print qq{before foreach loop:};
 print Dumper $_, \@array;
 foreach (@array) {
   print qq{in foreach loop before while loop: \$_ is $_};
   while (<DATA>) { print; }
   print qq{in foreach loop after while loop:  \$_ is },
         defined() ? $_ : 'UNdefined'
         ;
   }
 print qq{after foreach loop:};
 print Dumper $_, \@array;
 __DATA__
"
before foreach loop:
$VAR1 = 'foo';
$VAR2 = [
          'A',
          'B',
          'C'
        ];

in foreach loop before while loop: $_ is A
in foreach loop after while loop:  $_ is UNdefined
in foreach loop before while loop: $_ is B
in foreach loop after while loop:  $_ is UNdefined
in foreach loop before while loop: $_ is C
in foreach loop after while loop:  $_ is UNdefined
after foreach loop:
$VAR1 = 'foo';
$VAR2 = [
          undef,
          undef,
          undef
        ];
[download]

Consider the behavior with explicit localization:

>perl -wMstrict -MData::Dumper -le
"my @array = qw(A B C);
 $_ = 'foo';
 print qq{before foreach loop:};
 print Dumper $_, \@array;
 foreach (@array) {
   print qq{in foreach loop before while loop: \$_ is $_};
   { local $_;  while (<DATA>) { print; } }
   print qq{in foreach loop after while loop:  \$_ is },
         defined() ? $_ : 'UNdefined'
         ;
   }
 print qq{after foreach loop:};
 print Dumper $_, \@array;
 __DATA__
"
before foreach loop:
$VAR1 = 'foo';
$VAR2 = [
          'A',
          'B',
          'C'
        ];

in foreach loop before while loop: $_ is A
in foreach loop after while loop:  $_ is A
in foreach loop before while loop: $_ is B
in foreach loop after while loop:  $_ is B
in foreach loop before while loop: $_ is C
in foreach loop after while loop:  $_ is C
after foreach loop:
$VAR1 = 'foo';
$VAR2 = [
          'A',
          'B',
          'C'
        ];
[download]

All the usual suspects: Foreach Loops in perlsyn,  $_ (or  $ARG if you use English;) in perlvar, map, grep.

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