If you want to declare two my-variables, use either of the following:

my ($a, $b);
my $a; my $b;
my $a, my $b;
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By the way, the usage of the variables $a and $b for something other than sort is generally frowned upon. Probably that's why strict (if you use it) did not complain that the variable $b is not declared: Because it's a special variable and it's declared internally.