use strict;

Enable strict mode. There are no errors because of undefined vars because you used $a and $b, which are always defined, as they are used by sorting.
For the same reason, you choose @INC;

map{$a++}('g','r','e','p');

You set $a to 4.

$a=$a-$a--+$a++;

or written a little bit cleaner

$a=$a - $a-- + $a++;

But still rather hard to understand, because of the way auto-in/decrement worksin suffix mode:
What happens is (if I got it right):

$a=$a - $a-- + $a++;

$a is now 3, but 4 is substracted

$a=3 - 4 + $a++;

$a=-1 + $a++;

$a is incremented to 4, but 3 (the old value) is used in the addition

$a= -1 + 3

$a is now 2

$a++;

And now its 3

$b=int(rand($a));

sets $b to a value between 0 and 2.

$b++

increments it by one, so it is now 1, 2, or 3. Used to decide which virtue to print.

map {chomp;@INC=split(/\D/);push(@_,(int($INC$b/$b),
int($INC$b+$a/$b)));}(<DATA>);

Here you map over all elements of the DATA array, doing the following things:

chomp;

remove the line brake at the end of the line

@INC=split(/\D/);

Set @INC, by splitting the current line on non-numeric values, thereby removing all charaters from the stuff in DATA. This made me wonder, because first I thought you where using the characters hidden between the numbers as your data. But, as I then found out, you just tossed me a red herring that I gullibly swalloed.

push(@_,(int($INC$b/$b),int($INC$b+$a/$b)))

Now you modify your data structure. In the DATA-section, you stored the ascii values of each character (multiplied by either 1,2 or 3), like so (using the real characters): LIHamu (and these are NOT the 'L', the 'I', the 'H' etc you can see in the DATA section).
With this code segment, you first get item number $b (and divide by $b, so it is now the real ascii value), then you get $b+$a. As $a is 3, this gets the next value of the selected virtue. The shorter virtues are space-padded at the end.

map{printf("%c",$_)}@_;

Here you finally map over all values of @_, printing the character with the ascii value of $_. A nice way to prevent the much-overused chr. Voila.

Nice work! Especially as it's not just one more japh.

I espcially liked the auto-in/decrement stuff and the wrong track you laid in DATA.

--
#!/usr/bin/perl -w just another perl hacker
print+seek(DATA,$=*.3,@-)?~~<DATA>:$:__DATA__

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