...he says it's not an issue, due to the fact that the values are all 4-digit numbers.

But it is. Try it with these values to see:

my @one = ( 1234, 5678, 9012 );
my @two = ( 1234, 7891, 9001, 9002 );
my $bad = 5678;
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Output:

Use of uninitialized value in join or string at junk.pl line 21.
1234 5678 9012  9002
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