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#!/usr/bin/perl -w

...
@words = sort keys %{+{ map { !$common{$_ = lc $_} ? ($_, 1) : () } @w
+ords }};

print "@words\n";

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

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