Alternative:

my %counts;
my @commons = grep { ++$counts{$_} == @s }
              map { my %seen; $seen{$_}++ for /./sg; keys %seen } @s;
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or

use List::MoreUtils qw( uniq );
my %counts;
my @commons = grep { ++$counts{$_} == @s } map { uniq /./sg } @s;
[download]

Tested.