The string "Lentigo maligna melanoma [172.9]" contains a dot in the bracketed part, so the regexp doesn't match it. Therefore the variables $1 to $9 remain unaltered. (They are only set if there is a successful match.)

Try something like:

my ($diagnosisString, $diagnosisNumber) = ($diagnosis =~ /^(.*)\s\[(\d
+*)\]/);
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Or alternatively:

my $diagnosisString;
$diagnosisString = $1 if $diagnosis =~ /^(.*)\s\[(\d*)\]/;
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As a general rule, don't touch the regexp-related variables unless you know that the last match succeeded.

Update: thanks AnomalousMonk and mr_mischief for pointing out my missing $ sign in the second example. Fixed now.