$ 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]

##</code><code>##

$ 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]

##</code><code>##

$ 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]

##</code><code>##

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

        for my $i (1 .. $max) {
            say "\$f[$f] \$i[$i]";
        }
    }
'

##</code><code>##

$ 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.

##</code><code>##

$ 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};
    }
'