Check out "help perlop".

In the case of an empty regex, perl will use the last successfully-matched regex. So, in your case, it's attempting to use /(1)(2)/ on both of those strings and failing. If you do the following: 
$T = qr//;
$U = qr//;
$_ = "12";
/(1)(2)/;
my ($t, $u) = ($1, $2);
$t="12";
print $t =~ /$T/ ? "pass1, " : "fail1, ";
print $u =~ /$U/ ? "pass2\n" : "fail2\n";

[download]
You will see:

"pass1, fail2"

This functionality is... questionable.

In reply to Re: Empty qr// fails to match -- Is this known bug? by wokka
in thread Empty qr// fails to match -- Is this known bug? by chibiryuu

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