($a) = "abc" =~ /a(b)c/ or 'd';
print defined $a ? "defined as $a\n" : "undef\n"; # prints "defined as b"
($a) = "abc" =~ /a(x)c/ or 'd';
print defined $a ? "defined as $a\n" : "undef\n"; # prints "undef"

##</code><code>##

($a) = "abc" =~ /a(b)c/ or $a = "d";