/$pat1/ && /$pat2/;          # Sets $1 (twice).
/$pat1/ && /$pat2/;          # Sets $1.

##</code><code>##

/^(?=.*?$pat1)(?=.*?$pat2)/; # Sets $1 and $2.
/^(?=.*?$pat1)(?=.*?$pat2)/; # Doesn't set $1 or $2.

##</code><code>##

    $pat1 = "(foo)";
    $pat2 = "(bar)";
    /$pat1/ && /$pat2/;
    /^(?=.*?$pat1)(?=.*?$pat2)/;

    $pat1 = "(foo)";
    $pat2 = "(baz)";
    /$pat1/ && /$pat2/;
    /^(?=.*?$pat1)(?=.*?$pat2)/;