What is the significance of an underscore in Perl ($_, @_)?

user_113124 0
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I am new to Perl, and I used them like this

$_

foreach (@list) {
    print "$_\n";
}

@_

sub search {
    my ($no, @list) = @_;
}

How exactly do these underscore variables work in Perl? What are the other constructs where they are useful?

perl
3 Answers

This question always comes up, because it's Perl auto-magic (I made up that term).

foreach (@array_list){
  print $_ . "\n";
}

Is equivalent to:

foreach my $item (@array_list){
  print $item . "\n";
}

The difference is that when no variable is supplied for the loop, Perl automatically sets the default $_ to provide each item.

Similarly when calling subroutines (or methods - {subroutine in an object}) Perl puts the arguments to the subroutine in the @_ array, since arguments will always be a list.

On the flip side, when a built-in subroutine (or any well-written subroutine) requires arguments and is called with no arguments, it will usually function on $_ or @_ depending on if it's expecting one or many arguments. Some subroutines/methods (AKA functions) may work on either scalar or list values, and they usually default to scalar context and function on $_ by default. Sometimes the name makes it clear that it expects a list (array or hash) and then it defaults to @_ as its input.

Keep in mind, the flip side is talking about subroutine inputs, and their output depends on additional factors. In fact, subroutine outputs usually set $_ or @_ simply by returning a scalar value or list values.

For more on how to write a subroutine that returns values based on context check out wantarray and here's a good breakdown of it:

Advanced Subroutine Techniques

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