Hello PerlMonks,
I'm trying to remove lines from a file which have a duplicate id number but things don't seem to be working right. A sample data line is like so: 
lat="37.4192" lng="-122.0574" United States ID No: 1123631397

[download]
And here is the code: 
open (FILE,  "<:utf8", "input.txt");
my @lines = <FILE>;
my @uniq = ();
my @purge = ();
my %seen = ();

foreach $line (@lines) {
    my $id = $line =~ m/ID No: (\d+)/;
    if ($seen{$id}++){
        push (@uniq, $line);
        $new_uniq++;
        }
    else { 
        push (@purge, $line);
        }
open (MYFILE, ">:utf8", "data.txt");
print MYFILE @uniq;
open (PURGE, ">>:utf8", "purge.txt");
print PURGE @purge;

[download]
After the first run, the data in data.txt appears to have the correct result. However, I was expecting that purge.txt would contain all lines that were removed as duplicates but, it only contains one line. Subsequent runs _always_ remove the first line of data from input.txt and places it in purge.txt. Could someone please point out my error? I'm trying to teach myself Perl but for some reason I can't get a grasp on what's going wrong here.
Thanks!

In reply to Removing duplicate lines based on a match by carrerag

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