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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