# 1193725.pl
use strict; use warnings;
use Path::Tiny qw/ path /;
use List::Util qw/ uniq /;

my $dir = '.';
my $iterator = path( $dir )->iterator;
while( my $file = $iterator->() ) {
    next unless $file =~ /\.txt$/;
    my @lines = path( $file )->lines({ chomp => 1 });
    path( $file )->spew( join "\n", uniq @lines );
}

__END__
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Hope this helps!

Update: Removed example output as it was distracting (after rereading question and amending example).

The way forward always starts with a minimal test.

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