%common = map { lc $_, 1 }  qw/ the and a /;
@words = qw / hello Hello hello a The there /;

# lowercase all the words
@words = map { lc $_ } @words;
# shorter: @words = map { lc } @words;

# filter out common words
@words = grep { !common{$_} } @words;

# make a hash
%words = map { $_, 1 } @words

# get the sorted keys
@words = sort keys %words;