chomp (my @lines = <DATA>);

Who knows how that works, but it does. It's a perl idiom you need to know.

Yes, it’s a useful idiom — but surely not so mysterious?

1. my @lines =
   Declares a lexical array variable and initialises it (with a list of array elements).

2. @lines = <DATA>
   The LHS (left-hand side) of the assignment is an array, which gives list context to the expression on the RHS. When the readline or “diamond” operator <...> is placed in list context, it “reads until end-of-file is reached and returns a list of lines.” Lines are defined as the text between successive input record separators. The input record separator, stored in $/, “is newline by default.” See readline and General Variables in perlvar.

3. chomp(...);
   When chomp is given a list, it chomps (removes the input record separator from) each element in the list.

That wasn’t really so hard, was it? ;-)

Avoid writing $_ in your code.

Yes, avoid writing $_ explicitly where possible. But be aware that an explicit $_ is often needed; for example:

• my @doubles = map { $_ * 2 } @numbers;

• $couples{$_} eq 'Betty' && print for keys %couples;  # Find Betty's husband

Hope that helps,