Consider a very simple loop like this:

my @a;
for (qw/a b c/) {
   push @a, $_;
   print (join ', ', @a), "\n";
}
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In the first iteration, @a contains a, so the output is

a
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In the second iteration, @a contains a, b, so the total output is

a        # output from previous iteration
a, b     # output from current iteration
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In the third iteration it will print a, b, c, so you'll see a total of three a in the output - probably the duplicates you complained about.

If you move the print out of the loop, you just get a, b, c because that's what @a contains.