There is one important things to note about foreach and while loops, namely the localization of the loop variable.

The foreach loop will always localize its loop variable (That is $_ unless one is spesified). This can easily be verified:

$_ = 42;
foreach (1, 2, 3) {
   print "Inside loop: \$_ = [$_]\n";
}
print "Outside loop: \$_ = [$_]\n";
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This will print

Inside loop: $_ = [1]
Inside loop: $_ = [2]
Inside loop: $_ = [3]
Outside loop: $_ = [42]

The while loop however, does not localize anything. This can be seen from the following example

$_ = 42;
while(<FH>) {
   print "Inside loop: \$_ = [$_]";
}
print "Outside loop: \$_ = [$_]\n";
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If <FH> produces three lines containing the digits 1, 2 and 3 the output will be

Inside loop: $_ = [1]
Inside loop: $_ = [2]
Inside loop: $_ = [3]
Outside loop: $_ = []

It will also produce a warning if you remembered to turn on the -w flag. Why the warning you might ask. The reason is because the <FH> will return undef when it has reached the end of the file, and $_ is assigned this undef. And as we all know, perl will warn us if we use an undefined value (if the -w flag is used).

So then, what is it that happens in your program ? First, let's just simplify it a little bit (concentrating on the first foreach loop):

foreach (@test_array) {
    print qq|Before while: [$_]\n|;

    open(IN, $0) || die "No file: $!\n";
    while (<IN>) {  }
    close IN;

    print qq|After while: [$_]\n|;
}
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Now, what happens is that the foreach loop creates spesial references into the @test_array. (Spesial in the sense that when you change $_, you also change the corresponding element in the @test_array)

The next line should then print out the value of $_, which in your example would be "Element1". Then we open ourself, and loop over each line in the file. The while (<IN>) construct implicitply assigns the $_ without first localizing it. Thus we actually change the elements of the array @test_array.

If you instead had used a foreach loop to iterate over the file, this problem would not have occured, because of the fact that it would have localized the $_ variable for you. Or you could localize $_ yourself by writing local $_. Another possible solution is to use lexical variables both in the foreach loop and in the while loop

foreach my $elem (@test_array) {
    open(IN, $0) || die "No file: $!\n";
    while( defined(my $line = <IN>) ) {  }
    close IN;
}
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(The defined test isn't really necessary in newer perls, but I like to be explicit here).

"But", I hear you complain, "my while loop is in a different function, a completly new scope". Well, that is correct if you are talking about lexical scoping (variables declared with the my keyword are lexical variables). However, $_ is a dynamically scoped variable (this fact you can't change, you can't declare my $_. Here is a good tutorial explaining the difference between a lexical scope and a dynamic scope.) However, since $_ is dynamically scoped, it will live on in the new function only to be assigned by the while loop.

I hope this shed some light on the problem at hand.

Autark.

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