I still like eval "\$one =~ tr/\Q$two\E//cd" as part of a solution. But there is also this:

sub CommonChars {
    my $comm = shift @_;
    $comm =~ s/[^\Q$_\E]//g    for  @_;
    my %seen;
    return grep !$seen{$_}++, $comm =~ /(.)/gs;
}
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Which works for any number of strings.