my $N = ...;  # Number of lines before matching line.
my $PATTERN = "..."; # What you want to match.
my @buffer;
while (my $line = <>) {
    if (@buffer < $N) {
        push @buffer, $line;
        next;
    }
    if ($line =~ /$PATTERN/) {
        print $buffer[0];
    }
    shift @buffer;
    push @buffer, $line;
}