In Perl 6, $_ is known as the topic. It is lexically scoped (unlike in Perl 5), and is automatically assigned by topicalizers.
Some of the various topicalizers are for, given, CATCH (which is a special form of given), sub and -> (pointy sub), and bare closures.
What follows is an example of topicalizing blocks (in a simple ELIZA program):
for <> -> $line { # Both $line and $_ are <>
given $line { # $line aliased to $_ (as before)
when :iw/i love (\S+)/ {
print "Do you have further plans for $1?\n"
}
when :i/sex/ {
print "Tell me more about your sex life.\n"
}
when :i/eliza/ {
print "You talkin' bout me?\n"
}
}
}
The for and given alias to $_, and the pointy sub (-> in the first line) aliases to its argument.
The funny slashy things with colons are the regular expressions from Perl 6. The rest should be fairly self explanatory.