What do you mean by "do a Regex"? Do you want to match the lines agains regular expresisons, or do you want to modify the lines via substitution?

As usually, there are more ways how to do that. For example, if you are in a *nix environment, you can tac your file and process it from the last line. Or, you can remember the last two lines in case you want to process them:

my @lines;
while (<>) {
    push @lines, $_;
    shift @lines if @lines > 3;
}
if ($lines[2] =~ $regex1) {
    if ($lines[1] =~ $regex2) {
        if ($lines[0] =~ $regex3) {
            # Do your work here.
        }
    }
}
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