I like your solution. The regex could be different if OP really only wants to delete keys with single digits instead of multiple digits. I suggest using the arrow notion when using a hash reference. 
use strict;
use warnings;
use Data::Dumper;

my $hash = {
         'abc' => {
                 def => 1,
                 ghi => 2,
                 jkl => 3,
                },
        'mno' => {
                 pqr => 4,
                 stu => 3,
                 vwx => 5,
                },
         2   => {
                 def => 5,
                 stu => 2,
                },
         0   => {
                 def => 3,
                 stu => 1,
                }, 
         123 => {
                 xyzzy => "and nothing happens"
                },
};

#print Dumper $hash;

for my $key (keys %$hash) {
    delete $hash->{$key} if $key =~ /^\d$/
}

print Dumper $hash;

__END__
$VAR1 = {
          '123' => {
                     'xyzzy' => 'and nothing happens'
                   },
          'abc' => {
                     'def' => 1,
                     'jkl' => 3,
                     'ghi' => 2
                   },
          'mno' => {
                     'stu' => 3,
                     'pqr' => 4,
                     'vwx' => 5
                   }
        };

[download]

In reply to Re^2: Deleting hash entry by Marshall
in thread Deleting hash entry by nandymamith

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.