++ on the previous answers.

Since you seem to be trying to dive a bit further into perl subs, your last question may be answered using Prototypes. Especially, the _ prototype which takes the default variable $_ as a parameter if none is provided. This only works for functions though, not operators. If I use say (same as print except \n is automatically appended) which prints $_ if no argument is provided :

use v5.20; # use warnings and strict. Makes "say" available

sub add( $ _ ) # Two scalars, the second is $_ if not provided
{
  $_ = $_[0] + $_[1];
}

sub ans
{
  $_
}

$_ = 7 and say;

add 4, 5 and say; # prints 9
add 2 and say;    # prints 11
add -4 and say "The answer is: ", ans; # prints 7
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With operators instead of functions, the best you can do in perl 5 is $_ = 4+4; $_ += 5;