This is how you would normally define a sub:

sub something {
    ...
}
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This would be a sub stored in a variable:

my $thing = sub {
    ...
}
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So sub { $_[0] == $_[1] } is a sub stored in a variable.

Now what I think you're really asking is the $_[0] part. When you have a sub, the variable @_ contains all of your inputs to that sub. So if you call:

mysub( $one, $two );
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Then @_ holds $one and $two, or:

@_ = ( $one, $two );
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Since this is an array, you can get at the first variable with $_[0] and the second variable with $_[1]. Does that make sense?