G'day cormanaz,

"I don't understand what is setting $_ ..."

In the examples in the "module docs", $_ is being set in the outer loop and $i is being set in the inner loop. A quick demonstration:

$ perl -E '
    for ("a" .. "b") {
        for my $i (1 .. 2) {
            say "\$_[$_] \$i[$i]";
        }
    }
'
$_[a] $i[1]
$_[a] $i[2]
$_[b] $i[1]
$_[b] $i[2]
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Using a named variable, as you've done, is much safer. $_ is a global variable: it could be changed by something in "stuff my script is actually processing"; or, some later addition to your code could affect it.

$ perl -E '
    for my $f ("a" .. "b") {
        for my $i (1 .. 2) {
            say "\$_[$_] \$f[$f] \$i[$i]";
        }
    }
'
$_[] $f[a] $i[1]
$_[] $f[a] $i[2]
$_[] $f[b] $i[1]
$_[] $f[b] $i[2]
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Here's a highly contrived example of additional code changing $_:

$ perl -E '
    for my $f ("a" .. "b") {
        for my $i (1 .. 2) {
            say "\$_[$_] \$f[$f] \$i[$i]";
        }
        s/$/.bu/;
    }
'
$_[] $f[a] $i[1]
$_[] $f[a] $i[2]
$_[.bu] $f[b] $i[1]
$_[.bu] $f[b] $i[2]
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If you'd put "use warnings;" near the top of your code, you would have seen a series of "Use of uninitialized value $_ ..." warnings. This may have alerted you to the problem you describe.

While I appreciate that the code you provided was purely for demo purposes, I thought I'd point out that "my $max = $f;" is going to be a problem with real filenames (you may already be aware of this). You get no output from:

$ perl -E '
    for my $f ("a" .. "b") {
        my $max = $f;

        for my $i (1 .. $max) {
            say "\$f[$f] \$i[$i]";
        }
    }
'
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Again, the warnings pragma points to the problem:

$ perl -E '
    use warnings;

    for my $f ("a" .. "b") {
        my $max = $f;

        for my $i (1 .. $max) {
            say "\$f[$f] \$i[$i]";
        }
    }
'
Argument "a" isn't numeric in foreach loop entry at -e line 7.
Argument "b" isn't numeric in foreach loop entry at -e line 7.
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I strongly recommend that you include the warnings pragma in all of your code. It's lexically scoped: turn off selected warnings, in a limited scope, for special cases.

$ perl -E '
    use warnings;

    my @rgb = qw{#ff0000 #00ff00 #0000ff};
'
Possible attempt to put comments in qw() list at -e line 4.

$ perl -E '
    use warnings;

    my @rgb;
    {
        no warnings "qw";
        @rgb = qw{#ff0000 #00ff00 #0000ff};
    }
'
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