What does !$saw{$

nwkcmk has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I'm a amateur in Perl. I saw from web that following code is used remove duplicates in a array

undef %saw;
@out = grep(!$saw{$_}++, @in);
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Looks like !$saw{$_}++ is too much for me to digest. Hopefully someone can explain to me what this expression means.