#! /usr/bin/perl
use strict ;
use warnings ;
use Tie::File ;

my $filename = shift @ARGV ;
tie my @lines, 'Tie::File', $filename or die "Can't open file: $!" ;

for ( 0 .. $#lines ) {
    if ( $lines[$_] =~ /du/ ) {
        print map { $_ . $/ } @lines[$_-2..$_] ;
    }
}