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;