A nit (which I'm sure atcroft knows about) for the benefit of OP: it is important to remember from time to time that $_ is actually an alias to each element in the list being processed by a for loop (or map or grep). This is important because if you change $_ inside the loop the current list element's value is altered. Consider:

use strict;
use warnings;

my @array = qw(1 3 5 6);

++$_ for @array;
print "@array";
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Prints:

2 4 6 7
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Each of the elements in the array @array has been incremented by the for loop.