Update: You might want to read perldoc -q duplicate for a general discussion of using a hash to check for duplicates.

I know giving a complete solution is sometimes frowned upon here, but sometimes I just can't help myself. Here is the first script:

    #!/usr/bin/perl
    use strict;
    use warnings;

    my %nets;
    /^Net '([^']+)'/ 
        and not $nets{$1}++ 
        and print "$1\n" 
      while <STDIN>;
[download]

And the second:

    #!/usr/bin/perl
    use strict;
    use warnings;
    use File::Slurp;

    my $file2 = shift
        or die "Usage: $0 file2 < another-file";
    my $nets = join "|", map {chomp;$_} read_file($file2);

    /$nets/
        or print
      while <STDIN>;
[download]

And you can run the whole chain with something like:

    perl script1.pl < file1 > file2
    perl script2.pl file2 < another-file
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Update: just for fun, here's a version that will do everything in one step:

    #!/usr/bin/perl
    use strict;
    use warnings;
    use File::Slurp;

    my $file1 = shift
        or die "Usage: $0 file1 < another-file";
    my $nets = join "|", map { chomp;
                               /^Net '([^']+)'/ and $1 or ()
                             } read_file($file1);

    /$nets/
        or print
      while <STDIN>;
[download]

Which would be used like:

    perl script1+2.pl file1 < another-file
[download]