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
                   }
        };
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