Re: escaping the @ in an interpolating string

[Corion]

@$0 is seen as @{ $0 }, which takes $0 as a symbolic reference to the array named -e:

perl -e '@{q(-e)} = qw(Hello World); print "$0: @$0\n"'

-e: Hello World

If you enable strict, symbolic references are disallowed though:

perl -Mstrict -e '@{q(-e)} = qw(Hello World); print "$0: @$0\n"'

Can't use string ("-e") as an ARRAY ref while "strict refs" in use at 
+-e line 1.

[Danny]

Also, if you are unsure what $0 means see 'perldoc perlvar' and search for $0.

[marto]

Or

perldoc -v '$0'

See also: perldoc

[raygun]

I guess I should have been less specific!

@$0 is seen as @{ $0 }

The issue isn't limited to $0 following the @: no matter what variable follows it—whether it exists or not—the @ is not interpreted literally.

$ perl -e 'print "@$$\n";'

$ perl -e 'print "@$undefined_variable\n";'

$ perl -e 'print "@$]\n";'
]
$ perl -e 'print "@$(\n";'
(

Interestingly, sometimes the entire @$foo seems to be interpreted as a unit, printing nothing, and other times @$ seems to be interpreted as one unit, printing whatever follows it literally. In no case is the @ interpreted literally. And, of course, in all cases preceding the @ with a backslash solves that, and allows the following variable to be interpolated:

$ perl -e 'print "\@$$\n";'
@24477
$ perl -e 'print "\@$undefined_variable\n";'
@
$ perl -e 'print "\@$]\n";'
@5.030003
$ perl -e 'print "\@$(\n";'
@500 7 10 11 16 18 19 27 35 80 85 100 105 500

[Danny]

Those are all doing the same thing that Corion explained. They are inferring that $$, $undefined_variable etc are symbolic references to an array. If you 'use strict' it will give an error. Without 'use strict' you can do something like:

perl -e '@hi = (1,2,3), my $x = "hi"; print "@$x\n";'

and it will output:

1 2 3

[raygun]

Ah, it would've helped if I hadn't misread Corion's explanation. Yours managed to penetrate my thick skull. Thank you!

[NERDVANA]

More to the point, @$foo is perfectly useful syntax even with strict enabled:

use strict;
use warnings;

my $foo= [ 1, 2, 3 ];
for (@$foo) {
  say $_
}

[japhy]

In the case of those punctuation variables (e.g. $] and $(), Perl interprets "@$]" as "@$" and "]", and since @$ is empty, you get nothing, followed by the punctuation character.

[raygun]

Yeah, I could deduce from the output that it was doing that, but initially I couldn't figure out why it would parse those two differently from the others. But with the light shed by Corion and Danny, I figured it out: $] and $( expand into things that aren't legal variable names. So perl must, upon noticing that, deduce that a symbolic reference is not what it's seeing, and fall back on the alternate parsing you mention.