in reply to Re^5: Passing argument by reference (for a scalar)
in thread Passing argument by reference (for a scalar)

+ is the unary-+ operator. It does absolutely nothing but evaluate to the same thing its operand does. It even provides the same context as the one in which it's evaluated. 

+5        # Same as `5`
+"abc"    # Same as `"abc"`
+@a       # Same as `@a`, in both scalar and list context

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${ shift } is just a weird undocumented way of writing $shift. I didn't want that. I wanted to use the shift operator. ${ +shift } can't be parsed as a scalar named +shift, so it has the desired effect.

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Re^7: Passing argument by reference (for a scalar)
by sleet (Pilgrim) on Sep 11, 2024 at 15:50 UTC
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      It's documented to be allowed in string literals, but the OP is not using it in a string literal.

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Re^7: Passing argument by reference (for a scalar)
by etj (Priest) on Sep 11, 2024 at 04:36 UTC
    A truly useful use of + is for constructing hashes with map: 
    my %hash = map +($_=>undef), @list;

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    Without the +, Perl would think you wanted to call map with 2 args, not over @list.
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      > A truly useful use of + is for constructing hashes with map:

      > my %hash = map +($_=>undef), @list;

      For varying definitions of "useful". This needs 2 letters more than the usual notation

      my %hash = map {$_=>undef} @list; °

      update

      °) see choroba's reply why the parser will have trouble interpreting this.

      FWIW: I'd use a hash-slice @hash{@list} = () anyway

      Cheers Rolf
      (addicted to the Perl Programming Language :)
      see Wikisyntax for the Monastery

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        Which, in turn, sometimes needs another similar trick: 
        my %hash = map { "a$_" => undef } @list;   # syntax error at ... line 
+1, near "} @list"
my %hash = map {; "a$_" => undef } @list;  # OK.

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        map{substr$_->[0],$_->[1]||0,1}[\*||{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^*ARGV,3]
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        The "usual notation" you cite has the disadvantage that it has to create, then destroy, a scope for each element of @list, which is an unnecessary performance hit.
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