$ perl -Mstrict -Mwarnings -e '
    use autodie;
    use Tie::File;

    my $re = qr{^((\d+).+$)}s;

    my %data_for_part;

    open my $f1, "<", "pm_1045452_file1.txt";
    while (<$f1>) {
        /$re/;
        $data_for_part{$2} = $1;
    }
    close $f1;

    tie my @file2, "Tie::File", "pm_1045452_file2.txt";
    print $data_for_part{$_} for @file2;
    untie @file2;
'
3478749:AA:1D:AAA:DescriptionsY:C:2
3633731:AA:3E:AAA:DescriptionsZ:C:2

##</code><code>##

$ cat pm_1045452_file1.txt
3478748:AA:1D:AAA:DescriptionsX:C:2
3478749:AA:1D:AAA:DescriptionsY:C:2
3633731:AA:3E:AAA:DescriptionsZ:C:2

##</code><code>##

$ cat pm_1045452_file2.txt
3478749
3633731