#!/usr/bin/perl 
my $a = "123{12\n";
print "ACK non-digit!\n" if ($a =~ /[[:^digit:]]/);
print "ACK \\D!\n" if ($a =~ /\D/);
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Update: Looking over perlre I see how there could be some confusion. It describes, e.g., [:digit:] as "equivalent" to \d, but they are not quite equivalent, because, whereas \d may be used either inside or outside a regular Perl character class, [:digit:] can be used only within a Perl character class. This is what I mean:

  DB<1> $s = 123

  DB<2> x $s =~ /(\d)/g
0  1
1  2
2  3
  DB<3> x $s =~ /([\d])/g
0  1
1  2
2  3
  DB<4> x $s =~ /([:digit:])/g
  empty array
  DB<5> x $s =~ /([[:digit:]])/g
0  1
1  2
2  3
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(I realize that I'm using the term "character class" in two distinct ways, one to refer to the usual Perl regexp construcs, such as [0-6:-], and the other to refer to POSIX character classes; but in doing so I'm just following perlre's terminology.)