Equivalent code, with comments

use Data::Dump 'pp';

my @a = qw/2 1 3 4 5 4 3 3 4 5/;
my %seen;
my @result;

for ( @a ) {               #for each element in @a
    if ( not $seen{$_} ) { #if the element hasn't already been seen
        push @result, $_;  #add it to the result set
    }
    $seen{$_}++;           #note that the element's been seen
}

print join(",", @result), "\n";

pp \%seen;                 #{ 1 => 1, 2 => 1, 3 => 3, 4 => 3, 5 => 2 }
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Each value in %seen is a count of how many times the key's been seen

Update: Removed a misleading comment & corrected a typo
Update^2: replaced say with print for perl<5.10 compatability (as suggested by ww)