Are you asking what $_ is? Or what the idiom 0 + $variable is?

$_ is the current topic. It's the single item that gets passed to map every time through

0 + $variable (where your variable is $_) takes a string that contains a number, (like "7") and changes it into an actual number, (like 7). Basically, adding 0 "numifies" the string.

    -Bryan