Only strings can be hash keys. So when building %dict with regular expressions (the qr// constructs), they are converted to strings to become keys in the hash¹. For regular expressions, incidentally, these are strings that can be compiled back into the same regular expression they stemmed from - which is quite convenient.

So why did this not do anything to bichonfrisee74's input strings?

my %dict = (
    'brown'      => 'yellow',
    qr/int(\d+)/   => 'int',
    qr/float(\d+)/ => 'float',
);
[download]

I was scratching my head myself until I use'd re 'debug'. zwon happened to forget to escape the parentheses in his (intermediary) example. So if you use the (correctly written) qr// constructs below, things "work" as expected:

my %dict = (
    'brown'      => 'yellow',
    qr/int\(\d+\)/   => 'int',
    qr/float\(\d+\)/ => 'float',
);
[download]

---

¹The ?-xism used to confuse me until I realized this just explicitly states: turn off the x, i, s and m regex flags.