How does a multivalued hash work?

Question

Perl

While analyzing examples of working with hashes in perl,
I could not understand such an example of working with a multi-valued hash:

%ttys=();
open (WHO, "who|");
while(){
        ($user, $tty) = split;
        push(@ {$ttys{$user}}, $tty);
}
foreach $user (sort keys %ttys){
  print "$user: @{$ttys{$user}}\n"
}

I do not quite understand how the placement in the array occurs:
push(@ {$ttys{$user}}, $tty);
Please explain!

Answer 1

It is for this structure.
push takes two arguments, an array ( @{$ttys{$user}}) and a value to add ( $tty)
what kind of animal is that @{$ttys{$user}}?
$ttys{key}means that we are working with a scalar corresponding to the key key
@{$ref}getting an array by reference.
Together, it turns out like this: take the value from the ttys hash by the $user key and dereference the resulting link as an array.
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Answer 2

At one time, the book "Perl. Special Handbook" by S. Holzner helped me deal with such structures at the initial stage.
And by the way, your code is not very good. Compare with this:

#!/usr/bin/env perl

use strict;
use warnings;
use Data::Dumper;

my %ttys;
open my $WHO, '-|', '/bin/who' or die "open() error: $!\n";
while (<$WHO>) {
    my ($user, $tty) = split;
    push @{$ttys{$user}}, $tty;
}
close $WHO;
#print Dumper(\%ttys);
foreach my $user (sort keys %ttys) {
  print "$user: @{$ttys{$user}}\n";
}