Another way would be to use Parse::RecDescent.

#!/usr/bin/perl -w
use strict;
use Parse::RecDescent;

my $parser = new Parse::RecDescent (q{
   L: '0' L '1'
   L: ''
 });

my $string1 = "00111";
my $string2 = "0011";

print $parser->L(\$string1) && $string1 eq '' ? "" : "not ", "in L\n";
+ # not in L
print $parser->L(\$string2) && $string2 eq '' ? "" : "not ", "in L\n";
+ # in L
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I am a Parse::RecDescent newbie. Does someone know to write the evaluation in a better way?