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  1. #!/usr/bin/perl
  2. push @{$h{lc $_}}, $_ for <STDIN>;
  3. print "@{$_}" for grep{@$_ > 1} values %h;
Success #stdin #stdout 0s 17496KB
stdin
aB.txt
Ab.txt
xx
cd
cD
CD
stdout
aB.txt
 Ab.txt
cd
 cD