Well, my first thought was simply to write something that was simple, and not to worry about the speed. It depends on how many tens of million times this is going to happen before the speed even becomes an issue.

However, since you asked:

use Benchmark;
$a="Hello";
timethese(10_000_000, { eq => sub { return ($a eq "" || $a eq "?") },
                         regexp => sub { $a=~/^\??$/ } } );
[download]

Benchmark: timing 10000000 iterations of eq, regexp...
        eq:  7 wallclock secs ( 3.51 usr + -0.01 sys =  3.50 CPU) @ 28
+57142.86/s (n=10000000)
    regexp: 22 wallclock secs (10.15 usr +  0.13 sys = 10.28 CPU) @ 97
+2762.65/s (n=10000000)
[download]

--
Tommy
Too stupid to live.
Too stubborn to die.

if(!$foo || $foo =~ /\?/){
 # either its blank or there is a ? somewhere in it
}
[download]

if(!$foo || $foo =~ /\?/){
 # either its blank or there is a ? somewhere in it
}
[download]

actually, that would read: # it is blank, '0' or there is a '?' somewhere in it