Greetings jesuashok,

It's a very curious line, and others have explained what it does already; however, it has a couple negatives: It's not easily understood, and it's slower than a more easily understood alternative.

use strict;
use warnings;
use Benchmark qw(cmpthese);

cmpthese(
    100000,
    {
        'ahash' => sub {
            my $pat = {qw(a - b = c ~ d ^)}->{substr($0, 0, 1)};
        }, 
        'tr'    => sub {
            (my $pat = substr($0, 0, 1)) =~ tr/abcd/-=~^/;
        },
    },
);
[download]

The above benchmark returns this:

          Rate ahash    tr
ahash  75120/s    --  -86%
tr    541712/s  621%    --
[download]

It's a nice trick, but I wouldn't use it anywhere except in an obfu.