updated to correct language testing a few (simple - no refactoring or additional conditions, etc) additions to your code:

#!/usr/bin/perl 
my $a = "123{12\n";
print "ACK digit! per cazz posix\n" if ($a =~ /[:^digit:]/);
print "\npast cazz posix\n";

print "ACK digit! per ww-digit-posix\n" if ($a =~ /[^:digit:]/);
print "\tpast ww posix\n";

print "ACK digit! per ww-ISdigit-posix\n" if ($a =~ /[^:isdigit:]/);
print "\tpast ww ISdigit-posix\n";

print "ACK ISdigit! per cazz-ISdigit-posix\n" if ($a =~ /[:^isdigit:]/
+);
print "\npast cazz ISdigit posix\n";

print "ACK \\D!\n" if ($a =~ /\D/);
[download]

Output from CL execution of perl cazz.pl

past cazz posix
ACK digit! per ww-digit-posix
past ww posix
ACK digit! per ww-ISdigit-posix
past ww ISdigit-posix

past cazz ISdigit posix
ACK \D!

But:

print "ACK digit! per ww-ISdigit-posix\n" if ($a =~ /[[^:isdigit:]]/);
+  #changed; dbl brkts
[download]

changes output:
past cazz posix
ACK digit! per ww-digit-posix
past ww posix
past ww ISdigit-posix <!----

past cazz ISdigit posix
ACK \D!