use strict; use diagnostics;
$_ = "Before";
print "Before loop= $_\n";
while (<>){
    print "In while loop= $_\n";
    last;
}
print "After while loop= $_\n";

Output:
Before loop= Before
myinput  (entered from the keyboard)
In while loop= myinput

After while loop= myinput
[download]

A foreach loop does implicitly localize $_ (or whatever variable is being used as the loop variable), but $_ is not automatically localized for while(<>) loops. Here is the relevant entry from the perlop manpage:

Ordinarily you must assign the returned value to a vari- able, but there is one situation where an automatic assignment happens. If and only if the input symbol is the only thing inside the conditional of a `while' state- ment (even if disguised as a `for(;;)' loop), the value is automatically assigned to the global variable $_, destroy- ing whatever was there previously. (This may seem like an odd thing to you, but you'll use the construct in almost every Perl script you write.) The $_ variables is not implicitly localized. You'll have to put a `local $_;' before the loop if you want that to happen.

However, that last line is misleading: you'll need to wrap it all in a block, or localize $_ in the while condition to get the proper effect:

{
    local $_;
    while(<>){
        print;
    }
}

# or,

while(local $_ = <>) {
    print;
}
[download]

No, while doesn't use automatic localizing. Foreach does.

-- Randal L. Schwartz, Perl hacker

Which is helpful because this allows while loops inside the foreach loop to use the foreach $_. Correct?