Does anyone know what this $:$$$ means in Perl?

Yes, it is a syntax error.

use strict;
use warnings;
sub dothis ($:$$$) {
           ## rest of the code
}
__END__
Illegal character in prototype for main::dothis : $:$$$ at d:\tmp\test
+.pl line 3
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#!/usr/bin/perl

#can use prototype to get around passing hashrefs to subs 

my_sub(%hash)   #prototype for my_sub 

sub my_sub(\%) {
   my $hash_ref = shift;
   map { print "$_ => $$hash_ref{$_} \n" } keys %$hash_ref;
}
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You need to define the prototype before you can use the function.

sub my_sub (\%) { ... }

my_sub(%hash);
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Or if you want to define your function later:

sub my_sub (\%);

my_sub(%hash);

sub my_sub (\%) { ... }
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_____________________________________________________
Jeff[japhy]Pinyan: Perl, regex, and perl hacker, who'd like a job (NYC-area)
s++=END;++y(;-P)}y js++=;shajsj<++y(p-q)}?print:??;

Prototyping is of less value than you would expect; It's been discussed, so searching on prototype will show the reasons.

Prototyping IS sometimes of value if you want a routine to act like a built-in, automatically referencing hashes or arrays to modify them, or declaring a routine to take no arguments. It is totally incompatible with objects, unfortunnately.

I had a quick read through the perldoc man page for it and was thinking similar things. My thoughts at the time were "I'll have to come back and have a thorough read through this because I'm not seeing a great use of it right now".

Dean
The Funkster of Mirth
Programming these days takes more than a lone avenger with a compiler. - sam
RFC1149: A Standard for the Transmission of IP Datagrams on Avian Carriers

As TomDLux alluded to above, the only real point is to simulate perl "builtins". For example a grep replacement would look like sub map2(&@) (as I recall) and let you do stuff like  map2 { foo bar } @baz

I can recommend Tom Christiansens' rant against prototyping on why in general you want to stay away from it, even though the article is about 5 years old now.

FMTEYEWTK on Prototypes in Perl

It used to be available as an article on perl.com too, but not any more.