my @arr1 = (1, 3, 5, 2, 4, 6);
my @arr2;

# Works as expected.
@arr2 = sort { $a <=> $b } @arr1;
print join(' ', @arr2), "\n";

# Gives: Can't use "my $a" in sort comparison at ./sort_a.pl line 12.
my $a = 'whatever'; # just happens to be in scope of below sort block
@arr2 = sort { $a <=> $b } @arr1;
print join(' ', @arr2), "\n";