This another variant that uses qr// to check for the 'or' / '|' of all three digit sequences. I just boil it down to one regex here.

use strict;
use warnings;

my $dig3_regex_str = join '|',
    map { ($_, scalar reverse $_) } # 012 and 210
        map { join '', $_ .. $_+ 2 } 0..7;  # 012, 123, ...

my $dig3_regex = qr/$dig3_regex_str/;

# Test samples taken from other monks postings ...
my @strings =
  ( '10203040', '1234', '298761', '4562', '856423', 'a12b543c');

for my $string (@strings) {
    print "Bad string => $string\n"
      if $string =~ $dig3_regex;
}
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