Now I usually only use $_ in very small blocks of code, such as in a map or grep block. If the section of code large or is likely to grow then it is much clearer to assign to a named variable and this helps to document the code.

Today I was extending a bit of code that uses $_ absolutely everywhere and there was a point where the behaviour was a bit difficult to understand. I have always thought that you could assume that the magic of $_ made it work so that you could use it as if it were lexically scoped. So when you run this:

use strict;

for (1..3){
  print "BEFORE: " . $_;
  foo();
  print " AFTER: " . $_ . "\n";
}

sub foo{
 for(<DATA>){
   last;
 }
}

__DATA__
a
b
c
[download]

the fact that there is a call to foo() should make no difference to the value printed. And as expected, it doesn't:

BEFORE: 1 AFTER: 1
BEFORE: 2 AFTER: 2
BEFORE: 3 AFTER: 3

========== [C:\users\jake\code\komodo\test2.pl] run finished. ========
+==
[download]

So what's the problem? Well if you change foo() to read:

sub foo{
 while(<DATA>){
   last;
 }
}
[download]

ie use while instead of for, then the output is:

BEFORE: 1 AFTER: a

BEFORE: 2 AFTER: b

BEFORE: 3 AFTER: c


========== [C:\users\jake\code\komodo\test2.pl] run finished. ========
+==
[download]

Now I found this quite surprising. So my questions are:

• Is it supposed to do this?
• Is this behaviour documented?
• Why is there a big difference between for and while?

-- iakobski

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