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foreach $line (@lines) {
    my $id = $line =~ m/ID No: (\d+)/;
    if ($seen{$id}++){
# ...

foreach $line (@lines) {
    my ($id) = $line =~ m/ID No: (\d+)/;
    if ( ! $seen{$id}++){
# ...

open my $in_fh, '<:utf8', 'input.txt'
    or die "Can't read 'input.txt': $!";
my @lines = <$in_fh>;

open my $in_fh, '<:utf8', 'input.txt'
    or die "Can't read 'input.txt': $!";
...
close $in_fh       or die "Close failed for input: $!";
close $purge_fh    or die "Close failed for purge.txt: $!";
close $uniq_fh     or die "Close failed for data.txt: $!";