No special variable is available. Here are some options:

my @array = (1,2,3,4,5);

my $count = 0;
for (@array) { 
    print "index: ".$count++." contains $_\n";
}

for my $index (0..$#array) {
    print "index: $index contains $array[$index]\n";
}

# if you prefer thinking in terms of lines (like files)
for my $line (1..@array) {
    print "line: $line contains $array[$line-1]\n";
}

# reading from a filehandle
$count = 0;
while (<DATA>) {
    chomp;
    next unless $_;
    $count++;
    print "line: $count contains $_\n";
}

# kinda golfing
print "index: ".$index++." contains $_\n" for @array;

__DATA__
1
2
3
4
5
[download]

cheers

tachyon