Filtering, mapping, and numeric order

The idea

Enter with list context and blocks understood. grep retains values for which its block is true, while map produces transformed values. Both commonly use $_ for the current item. Treat that item as read-only unless modifying the source is intentional, because these operations can expose aliases. A pipeline should make it clear which stage selects data and which stage changes its representation.

sort without a comparison block uses string ordering, which may place ten before two. Use the numeric spaceship comparison $a <=> $b when ordering numeric values. The special sort variables $a and $b supply the compared pair. A comparator must consistently describe the requested order; do not use a boolean equality expression as if it were a three-way comparison. The example sorts an independent result list for display.

use strict; use warnings;
my @values=(10,3,2,7);
my @result=sort {$a <=> $b} map {$_ * 2} grep {$_ >= 3} @values;
print join(",",@result),"\n";

6,14,20

Filtering keeps ten, three, and seven. Doubling gives twenty, six, and fourteen, then numeric sorting orders them ascending.