This may probably be what you are looking for:

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,
                },                
};

print Dumper $hash;

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

print Dumper $hash;
[download]

Output:

$ perl delete_hash.pl
$VAR1 = {
          'mno' => {
                     'pqr' => 4,
                     'vwx' => 5,
                     'stu' => 3
                   },
          '0' => {
                   'def' => 3,
                   'stu' => 1
                 },
          'abc' => {
                     'def' => 1,
                     'jkl' => 3,
                     'ghi' => 2
                   },
          '2' => {
                   'def' => 5,
                   'stu' => 2
                 }
        };
$VAR1 = {
          'mno' => {
                     'pqr' => 4,
                     'vwx' => 5,
                     'stu' => 3
                   },
          'abc' => {
                     'def' => 1,
                     'jkl' => 3,
                     'ghi' => 2
                   }
        };
[download]