I'm not sure how I'd count this, as it does the printing, also.

#!/usr/bin/perl -l
$_ = shift;

print"$_ is",(grep$_>1,map++$c{$_},split//)?"n't":'',' an isogram';
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Output:

$ perl isogram.pl greet
greet isn't an isogram

$ perl isogram.pl isogram
isogram is an isogram

$ perl isogram.pl discrete
discrete isn't an isogram
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Update: I wouldn't count it, as /(.).*\1/ is much shorter.

Update 2 I'd still count it as the shortest of the non-regex versions. And here it is as a sub for easier counting:

sub is_iso {
(grep$_>1,map++$c{$_},split//,shift)?1:0
}


$_ = shift;
print "$_ is", ( is_iso($_)? "n't" : '' ), ' an isogram';
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s**lil*;  $*=join'',sort split q**;  s;.*;grr; &&s+(.(.)).+$2$1+; $; =
qq-$_-;s,.*,ahc,;$,.=chop for split q,,,reverse;print for($,,$;,$*,$/)
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