@saw{ @arr } = (); is a hash slice assignment. The following three snippets are functionally equivalent:

@saw{ qw( a b c ) } = ( 1, 2, 3 );
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( $saw{a}, $saw{b}, $saw{c} ) = ( 1, 2, 3 );
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$saw{a} = 1;
$saw{b} = 2;
$saw{c} = 3;
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Update: By the way, your solutions do not preserve the order of the items. If that was a requirement, you'd could use

my @arr = (4, 1, 6, 4, 3, 7);
my %saw;
my @new_arr = grep !$saw{$_}++, @arr;
print @new_arr;
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