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

##</code><code>##

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

##</code><code>##

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

##</code><code>##

open my $in_fh, '<:utf8', 'input.txt'
    or die "Can't read 'input.txt': $!";
open my $purge_fh, '>>:utf8', 'purge.txt'
    or die "Can't append to 'purge.txt': $!";
open my $uniq_fh, '>:utf8', 'data.txt'
    or die "Can't write 'data.txt': $!";

my %seen;

while ( my $line = <$in_fh> ) {
    my ($id) = ($line =~ /ID No: (\d+)/);
    if ( ! $seen{ $id }++ ) {
        print $uniq_fh, $line;
        $new_uniq++;
    }
    else {
        print $purge_fh, $line;
    }
}

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: $!";