#!/usr/bin/perl

use strict;

my ($prev, $next);
while (my $line = <DATA>) {
    if ( $next ) {
        print "$line";
        $next = undef;
    }
    if ( $line =~ /hello/ ) {
        print "$prev" if defined( $prev );
        print "$line";
        $next++;
    }
    $prev = $line;
}

__DATA__
test1
test2
hello
test3
test4