49, but then again I suck at golf.

sub isogram {
#234567890#234567890#234567890#234567890#234567890
my%h=map{($_,1)}split//,$_[0];keys%h==length$_[0]
}

print "$_ is ", (isogram($_) ? '' : 'not '), "an isogram\n" for @ARGV;
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Duh, johngg had the right idea, except he wasn't greedy enough (nor was I. What's with the $_[0]? Sheesh.):

sub isogram {
#23456789
/(.).*\1/
}

print "$_ is ", (isogram() ? '' : 'not '), "an isogram\n" for @ARGV;
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That makes for 9.

Very nice, but your logic is reversed.  Remember, an isogram does NOT have any letters duplicated.

To keep it golfed, either change the caller of the subroutine:

print "$_ is ", (isogram() ? 'not ' : ''), "an isogram\n" for @ARGV;
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or, better yet, the subroutine name:

sub not_isogram {
    # ...
}
print "$_ is ", (not_isogram() ? '': 'not '), "an isogram\n" for @ARGV
+;
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---

s''(q.S:$/9=(T1';s;(..)(..);$..=substr+crypt($1,$2),2,3;eg;print$..$/