qw(a - b = c ~ d ^) creates a list: "a", "-", "b", "=", "c", "~", "d", "^". Put that in braces, and you are initialising an anonymous hash. Use the arrow to dereference that hash. The brace to the right shows what key to use to dereference the hash. The key to use is the substring of $0 (the program name) - starting at position 0, length of 1.

It's like this:

my %underline = (
  a => '-',
  b => '=',
  c => '~',
  d => '^',
);
my $first_letter = substr($0, 0, 1);
my $pat = $underline{$first_letter};
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It's a cute trick, but I wouldn't recommend learning how to code from it ;-)