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my @a = map { transformation($_) } @b;
# same as
my @a;
push @a, tranformation($_) for @b;
@a

my @a = grep { condition($_) } @b;
# same as
...
   push @b, $_ if condition($_);
}
@a

my @even = grep $_ % 2 == 0, @numbers;
my @odd  = map $_ * 2 +1,    @numbers;

my @numbers = grep /^[0-9]+$/, @words;