That is dereferencing the array reference in $_ e.g

my @aoa = ( ['a' .. 'c'], ['d' .. 'f'], ['g' .. 'i'] );

print "[", @$_, "]\n" for @aoa;

__output__

[abc]
[def]
[ghi]
[download]

That iterates through @aoa aliasing each element to $_ and then the array reference in $_ is being dereferenced in the print statement. So to access individual elements we just dereference them like so

my @aoa = ( ['a' .. 'c'], ['d' .. 'f'], ['g' .. 'i'] );

print "first element - ", $_->[0], "\n" for @aoa;

__output__

first element - a
first element - d
first element - g
[download]

See. tye's References quick reference and perlreftut for a good starter on references and dereferencing.
HTH

_________
broquaint