A bit of code to demonstrate how the ambiguity between auto-deferencing hashrefs and arrayrefs can be caught.

hashref-arrayref.pl

#! /usr/bin/env perl

# a standard prefix for modern Perl
use 5.014;                                              # implies both `use strict` and `use feature :5.14`
use autodie qw< :all >;
use warnings FATAL => 'all';


my $arrayref = [ qw< foo bar baz bmoogle > ];
my $hashref = { map { $_ => 1 } @$arrayref };

say 'hash:  ', join(', ', keys $hashref);               # works fine
say 'array: ', join(', ', keys $arrayref);              # also works fine


my $corresponding_arrayref = $hashref;                  # oops!
foreach (keys $corresponding_arrayref)
{
    say $arrayref->[$_];                                # kaboom!
}