Of course, if you make the same stupid mistake twice (or any even number of times) in the same hash initialization, warnings won't help you:

c:\@Work\Perl>perl -MData::Dump -le
"use warnings;
 use strict;
 ;;
 my %hash = (
   'accents' => 'utf-8',
   'accounting' => 'money',
   'bank' => 'money'.
   'ing' => 'money',
   'foo' => 'bar'.
   'perls' => 'perl',
   );
 dd \%hash;
"
{
  accents    => "utf-8",
  accounting => "money",
  bank       => "moneying",
  barperls   => "perl",
  money      => "foo",
}
[download]

Because Perl so transparently interconverts numeric and string values, if the hash only contains literals, it may be advantageous to use  qw// to get rid of operators altogether:

c:\@Work\Perl>perl -MData::Dump -le
"use warnings;
 use strict;
 ;;
 my %hash = qw(
   accents utf-8
   accounting money
   bank money
   ing money
   foo 1
   perls perl
   );
 dd \%hash;
"
{
  accents => "utf-8",
  accounting => "money",
  bank => "money",
  foo => 1,
  ing => "money",
  perls => "perl",
}
[download]

Now there can be no confusion between the  , and  . operators. (Update: Of course, this trick only works for strings that have no embedded whitespace!)

BTW: In case you should feel that my characterization of this mistake as "stupid" is a bit harsh, don't worry: I'm a member of the "Been There, Done That, Got The Scars To Prove It" club too.