In map, $_ is a localised reference. Thus it doesn't clobber $_ my @array = qw/one two three/; $_ = "Hello"; say; map { tr/a-z/A-Z/ } @array;

In map, $_ is a localised reference. Thus it doesn't clobber $_ my @array = qw/one two three/; $_ = "Hello"; say; map { tr/a-z/A-Z/ } @array; say; prints.... Hello Hello However remember it's a reference so using tr// will update @array... say "@array"; # => ONE TWO THREE