Regexes aren't always the answer. This solution uses substr and the magical autoincrement properties of strings.

#!/usr/bin/perl -w

use strict;

while (<DATA>) {
  chomp;

  my $card = substr $_, 0, 1;

  my $run = 1;
  foreach my $x (1 .. 4) {
    ++$card ne substr $_, $x, 1 and $run = 0, last;
  }

  print "$_ - ", $run ? 'yep' : 'nope', "\n";
}

__DATA__
abcde
abcef
fghij
acghm
[download]

Update: Argh! Or either of the previous answers would probably be better :(

"The first rule of Perl club is you do not talk about Perl club."
-- Chip Salzenberg