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my @a = qw( a b c );
my @b = map { $_ = uc($_); $_ } @a;
say "@a";  # A B C     <-- You clobbered @a!!!
say "@b";  # A B C

my @a = qw( a b c );
my @b = apply { $_ = uc($_);} @a;
say "@a";  # a b c
say "@b";  # A B C