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
The input file I used:
$ cat pm_1053380_data.txt
line1
line2
line3
line4
line5
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).
-
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