G'day httpd,

It's unlikely that you really want a regex; however, if you do, here's one way to do it:

$ cat pm_1027453.dat
line1
line2
line3
[download]

$ perl -Mstrict -Mwarnings -E '
    say do { local $/; <> } =~ /(.*)\Z/m;
' pm_1027453.dat
line3
[download]

You've already been given non-regex solutions. Here's another using Tie::File:

$ perl -Mstrict -Mwarnings -E '
    use Tie::File;
    tie my @lines, q{Tie::File}, q{./pm_1027453.dat} or die $!;
    say $lines[$#lines];
    untie @lines;
'
line3
[download]

You may want to look at Benchmark to see which solution best suits your application.