Good call.

I was first thinking of another pitfall you have to watch out for, at least with strict off, and that is when while populating the hash, some entries may not be a ref or undef, but a string. In that case, Perl will go weird on you. For example:

use Data::Dumper;
no strict;
%hash = ( a => 'x123' );
$hash{a}{b} = 'data';
print Dumper \%hash;
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which produces:

$VAR1 = {
          'a' => 'x123'
        };
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So, where did your data go to? It went into the global variable %x123, via a symbolic reference ${$hash{$a}}{b}.

print Dumper \%hash, \%x123;
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produces

Name "main::x123" used only once: possible typo at Z:\test.pl line 6.
$VAR1 = {
          'a' => 'x123'
        };
$VAR2 = {
          'b' => 'data'
        };
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The "used only once" warning is a compile time warning, but using the symbolic reference (once) doesn't produce any warnings at all.