Yeah, like the other people said, your code has some serious fundamental problems... May need to hit an intro to perl tutorial or something... But, for now:

use strict;
use warnings;
use Data::Dumper;

open (FH, "<data.txt");
my (%hash);
while (my $line = <FH>) {
    my $key = lc($line);
    $hash{$key} = $line;
}
close(FH);

open (FH, ">hash.txt");
print FH Data::Dumper->Dump([%hash]);
close(FH);
[download]

That should work for you.