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;
}
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-- Randal L. Schwartz, Perl hacker

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