$cn =~ /^(0|5|6)0+\Q$bn\E$

You never know with demerphq's future optimizations in regexp speed, but I'm quite sure that for now,

/^[056]0+/
[download]

will be faster than

/^(?:0|5|6)0+/
[download]

Benchmark code:

use Benchmark 'cmpthese';
my @list = ( '00012345', 'D123470', '0000123450', '0000023456', '50000
+123990' );
cmpthese -3, {
   charclass => sub { grep /^[056]0+/, @list },
   alt => sub { grep /^(?:0|5|6)0+/, @list },
};
[download]

Benchmark result:

Benchmark: running alt, charclass, each for at least 3 CPU seconds...
       alt:  3 wallclock secs ( 3.00 usr +  0.00 sys =  3.00 CPU) @ 50
+5561.00/s (n=1516683)
 charclass:  3 wallclock secs ( 3.20 usr +  0.00 sys =  3.20 CPU) @ 53
+4972.53/s (n=1713517)
              Rate       alt charclass
alt       505561/s        --       -5%
charclass 534973/s        6%        --
[download]

So it is indeed 5-6% faster, depending how you look at it.