>>600
>607はあまりにも酷かったので、修正
#!/usr/bin/env perl
use List::MoreUtils;
@A = qw( 2019/07/05 2019/07/10 2019/07/15 2019/07/20 );
%A = map {$_, A} @A;

@B = qw( 2019/07/08 2019/07/10 2019/07/20 );
%B = map {$_, B} @B;

@C = qw( 2019/07/15 2019/07/20 2019/07/25 );
%C = map {$_, C} @C;

for $date ( List::MoreUtils::uniq (@A, @B, @C) ){
for $someone_is_free_at (\%A, \%B, \%C){
push(@free_men, $$someone_is_free_at{$date}) if $$someone_is_free_at{$date};
}
print "@free_men are free at $date\n" if (@free_men > 1);
@free_men = ();
}


出力
A B are free at 2019/07/10
A C are free at 2019/07/15
A B C are free at 2019/07/20