Do you know what "map" does? If not, you might take a look at the documentation.

Let's split the code up a bit to make it easier to follow.

my $keys = '1 2 3 4 5 6';

# this splits your input data and stores the split
# up elements in an array
# So @key_list ends up containing
#   (1, 2, 3, 4, 5, 6)

my @key_list = split /\s+/, $keys;

# Here's where it gets interesting.
# The map block is called once for every element in
# the input list and returns the list that is created
# by the block.
# In this case each element is converted to a two-element
# list, where the first element is taken from @key_list
# and the second element is the value "undef". So, for
# example, the first element from @key_list returns the
# two-element list (1, undef). All of this little lists
# are stored together in @mapped_list. So @mapped_list
# will contain:
#   (1, undef, 2, undef, 3, undef, 4, undef,
#    5, undef, 6, undef)

my @mapped_list = map { $_ => undef } @key_list;

# And finally we use standard list assignment to assign
# our array to a hash. The even indexed elements (indexes
# 0, 2, 3, etc) become the keys and the old indexed
# elements (1, 3, 5, etc) become the values.
# It's just like doing:
#   %hash = (1 => undef, 2 => undef, 3 => undef, ...)

my %hash = @mapped_list;
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Does that help at all?