my $re1 = qr/a/;
my $re2 = do { use if $ENV{FOO_DEBUG_REGEX},
    qw/re Debug EXECUTE/; qr/b/ };
my $re3 = qr/c/;
my $str = "ab";
$str =~ $re1;
$str =~ $re2;
$str =~ $re3;