Update: It does work when I get the syntax of the negative lookahead assertion correct. (?!...) not (?!=...)!

Try this:

perl -nle"/^(.)(?!\1)(.)(?!\1|\2)(.)(?!\1|\2|\3)(.)(?!\1|\2|\3|\4)(.)\
+3(?!\1|\2|\3|\4|\5)(.)(?!\1|\2|\3|\4|\5|\6).$/ and print" <words
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Expanded out that regex is:

/^                              ## With the $, exactly 8 chars only
    (.)                         ## Any char as \1
    (?!\1)(.)                   ## Any char as \2 except \1
    (?!\1|\2)(.)                ## Any char as \3 except \1 or \2
    (?!\1|\2|\3)(.)             ## Any char as \4 except ...
    (?!\1|\2|\3|\4)(.)          ## Any char as \5 except ...
    \3                          ## Only whatever is in \3
    (?!\1|\2|\3|\4|\5)(.)       ## Any char as \6 except ...
    (?!\1|\2|\3|\4|\5|\6).      ## Any char except any char we've alre
+ady seen
$/x
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That begs to be generated from some kind of shorthand spec. and actually, you used such a spec in your question.

'ABCDECFG' is a perfect spec. once you think of those letters as placeholders rather than literal characters.

Update: And here is a generator:

#! perl -slw
use strict;

my $spec = shift or die 'No spec supplied!';

my $re = '';
my %tally;
my $i = 1;
for my $c ( split '', $spec ) {
    if( exists $tally{ $c } ) {
        $re .= '\\' . $tally{ $c };
    }
    else {
        if( $i == 1 ) {
            $re .= '(.)';
        }
        else {
            $re .= '(?!' 
                . join( '|', map{ '\\' . $_ } values %tally ) 
                . ')(.)';
        }
        $tally{ $c } = $i++;
    }
}
$re = qr[^$re$];
print $re;

m[$re] and printf $_  while <>;
__END__
C:\test>wordSolver ABCDECFG words
(?-xism:^(.)(?!\1)(.)(?!\1|\2)(.)(?!\1|\3|\2)(.)(?!\1|\4|\3|\2)(.)\3(?
+!\1|\4|\3|\5|\2)(.)(?!\6|\1|\4|\3|\5|\2)(.)$)
abednego
abscised
airborne
...
whisking
worker's
writhing
ziegfeld
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I wonder if the golfers could reduce that to a one-liner?