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The unary + operator in Perl has a very specific and important purpose but it’s quite different from it’s behavior in languages like JavaScript.

In Perl, it’s not a numeric conversion operator.

It’s primary technical aspect is context disambiguation.

Enforcing Scalar Context

Perl has a fundamental concept of context: scalar context and list context.

The result of an operation can change dramatically depending on the context it’s evaluated in.

The unary + operator is used to force an expression to be interpreted in scalar context.

This is most commonly needed when you have a function or an operator that expects a scalar argument but the argument you’re providing is ambiguous and could be interpreted as a list.

The unary + resolves the ambiguity by signaling that what follows is an expression, not a hash reference.

use Data::Dumper;

sub bad {
    my %hash;
    $hash{ shift } = 'value';   # Ambiguous
    return \%hash;
}

sub good {
    my %hash;
    $hash{ +shift } = 'value';  # Unambiguous
    return \%hash;
}

print Dumper(bad('key'));       # { 'shift' => 'value' };
print Dumper(good('key'));      # { 'key'   => 'value' };

What unary + operator is NOT in Perl?

It’s critical to understand that Perl's unary + operator does not convert data types.

a) It does not convert a string to a number.
b) It does not have any mathematical effect on numbers.

my $string = "123abc";
my $number = +$string;     # This does NOT convert the string to 123!
print $number;             # Still prints "123abc"

my $num = 5;
my $result = +$num;        # $result is just 5. The + is a no-op here.

If you need to convert a string to a number, you would typically just use the string in a numeric context (like a mathematical operation) and Perl will do the conversion for you.

my $string = "123abc";
my $number = 0 + $string;  # Numeric context
print $number;             # Now prints "123"

Ambiguous blocks with map

The map function takes either a block {...} or an expression.

The parser has to figure out which one you’re using.

map { $_ * 2 } @array;     # the braces are a block for map

This is also clear - no braces, just an expression.

map $_ * 2, @array;        # no braces, just an expression

The ambiguity arises when you want to use a complex expression that includes a dereference or another block.

More Examples

my @nums = (1, 2, 3);

my @wrong = map  ($_ => $_ * 2), @nums;    # ('0')
my @right = map +($_ => $_ * 2), @nums;    # (1,2,2,4,3,6)

The map ($_ => $_ * 2), @nums is parsed differently than you might expect.

In Perl, map EXPR, LIST expects a single expression.

The parentheses here ( $_ => $_ * 2 ) don’t behave as a list.

Instead, Perl interprets it as the first element of map being a void context, which evaluates to 0.

Whereas in map +($_ => $_ * 2), @nums, the unary + operator forces the parentheses to be treated as a list expression not a block or ambiguous expression.

Now map sees the correct list ($_ => $_ * 2) for each element of @nums.

Here’s one more example:

my @names = ('Joe','Blog');

my @refs_block = map {{ name => $_ }} @names;   # ({'name' => 'Joe'}, {'name' => 'Blog'})
my @refs_plus  = map +{ name => $_ }, @names;   # ({'name' => 'Joe'}, {'name' => 'Blog'})

The unary + operator applies to what follows immediately, it’s saying "treat the next token as the start of an expression"

It’s purely syntactic at runtime, the unary + operator has no effect on the value.

my $href = +{ key => 'value' };  # Same as { key => 'value' }

This pattern appears with other functions that take blocks or expressions, like grep.

grep +{ $_->{active} }->{value}, @users;

If your expression doesn’t contain ambiguous braces then you don’t need the unary + operator.

These are fine without unary + operator.

map $_->{name}, @array_of_hashrefs;          # No braces at all
map $some_hash{$_}, @keys;                   # No ambiguous braces
map [ $_->{x}, $_->{y} ], @points;           # Square brackets are unambiguous

With map, unary + operator serves as a syntactic disambiguator that tells Perl to interpret {...} as a hash reference constructor (an expression) rather than as a map block.

It’s a crucial tool for writing correct Perl code when you need to return hash references or other brace-delimited constructs from map.

Happy Hacking !!!