>>600 Perl5

@a = map{sprintf"2019/07/%02d",$_} 1..31;
$h{$_}{A}=1 for qw{2019/07/05 2019/07/10 2019/07/15 2019/07/20};
$h{$_}{B}=1 for qw{2019/07/08 2019/07/10 2019/07/20};
$h{$_}{C}=1 for qw{2019/07/15 2019/07/20 2019/07/25};
@b = grep{1<keys %{$h{$_}}} @a;
print "$_ @{[keys %h{$_}]}\n" for @b;


実行結果
~ $ perl 14_600.pl
keys on reference is experimental at 14_600.pl line 6.
2019/07/10 B A
2019/07/15 C A
2019/07/20 B A C