This uses transliteration to map a certain set of characters to
another set. The source set is comprised of the ASCII characters
from '0' to '>'. If you look at the source string, you'll notice that
it's comprised entirely of characters within the source set.
The tr takes the source set and maps it onto the destination
set, which is ' LEOR!AUBGNSTY'; for each character in the source
set, it gets mapped to the corresponding character in the dest
set. For example:
'0' => ' '
'1' => 'L'
'2' => 'E'
'>' => 'Y'
It's just like a substitution cipher (I think that's the right name).
You can do it manually: go through the source string, the
string in $_, which is
and replace each character by the corresponding character in
the destination set. '6' is turned into 'A', '1' is turned into 'L', '1'
is turned into 'L', etc. And right there you have 'ALL'.
Make sense? The key point here is to know what tr does, so
you should read the docs for that if you're still not sure how