For each iteration, only one of the if/else branches is executed.

If the items shows up in both arrays, the two array references must be references to the same array. Compare:

# two distinct array refs:
$ perl -wE 'my $a = [1]; my $b = [1]; push @$a, 2; say for @$b'
1
# same array ref twice:
$ perl -wE 'my $a = [1]; my $b = $a; push @$a, 2; say for @$b'
1
2
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In the second case, both $a and $b point to the same array reference, so that pushing to one adds the value to the other.