Further to tobyink's post:

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

The neat thing about the expression used in this statement is that it evaluates to a list, which in scalar context evaluates to the number of items in the list. In the scalar context of an if conditional expression, the empty list returned by a failing match like the one above evaluates to zero, and zero is false. Thus, it's possible to immediately process the components captured from a successful match or to handle a failed match.

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $d ('aaaa 987', 'no joy',) {
   printf qq{'$d': };
   if (my ($x, $y) = $d =~ m{ \A (a+) \s+ (\d+) \z }xms) {
     print qq{match: x is '$x'; y is '$y'};
     }
   else {
     print 'no match';
     }
   }
"
'aaaa 987': match: x is 'aaaa'; y is '987'
'no joy': no match
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