use autodie;
open my $in, "<", "your-file";
open my $out, ">", "your-file.tmp";
while (<$in>) {
    print $out $_;
    if (/USER\s+NOTE/i) {
        print $out "@$global_variables_list\n";
        last;
    }
}
while (<$in>) {
    print $out $_;
}
close $out;
close $in;
rename "your-file.tmp", "your-file";