G'day rm,

Welcome to the monastery.

You just need to keep a sufficient history of previous lines so that you go back to the previous nth line. Here's one way to do it:

$ perl -Mstrict -Mwarnings -Mautodie -le '
    my $file = "pm_1053380_data.txt";
    my ($match, $before) = @ARGV;
    my @previous;

    open my $fh, "<", $file;

    while (<$fh>) {
        chomp;

        if ($_ eq $match) {
            print $previous[0];
        }

        push @previous, $_;
        shift @previous if @previous > $before;
    }
' line4 2
line2
[download]

The input file I used:

$ cat pm_1053380_data.txt
line1
line2
line3
line4
line5
[download]

Notes:

• You'll need to add some validation to check that you can actually look back the number of specified lines.
• I see you had chop and then commented it out. You probably want chomp (as I used).