Under use warnings; it gives a

Odd number of elements in hash assignment at  ...
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warning so define $SIG{ __WARN__ } to take some action.

Update: Here's a code example.

use strict;
use warnings;

$SIG{ __WARN__ } = sub { die q{Got a warning} };
my %hash = (
            'accents' => 'utf-8',
            'accounting' => 'money',
            'bank' => 'money'.
            'ing' => 'money',
            'perls' => 'perl',
        );
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Update 2: Further updated code as per Camel Book, 4th edn. pp 1041 so as to get the actual warning message

use strict;
use Carp qw{ carp croak confess cluck };
use warnings;

$SIG{ __WARN__ } = sub { confess q{Got a warning: @_} };
my %hash = (
            'accents' => 'utf-8',
            'accounting' => 'money',
            'bank' => 'money'.
            'ing' => 'money',
            'perls' => 'perl',
        );
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Output

Got a warning: @_ at xxxzzz line 5.
        main::__ANON__('Odd number of elements in hash assignment at x
+xxzzz line 6.\x{a}') called at xxxzzz line 6
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Update 3: The sub { confess q{Got a warning: @_} }; should of course have been sub { confess qq{Got a warning: @_} }; so that the @_ interpolates. The output then becomes

Got a warning: Odd number of elements in hash assignment at xxxzzz lin
+e 6.
 at xxxzzz line 5.
        main::__ANON__('Odd number of elements in hash assignment at x
+xxzzz line 6.\x{a}') called at xxxzzz line 6
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Thus the perils of rushing to post before dashing out for an appointment :-/