my @lines = 1 .. 20;
my @interesting = ( 1, 3, 6, 10 , 20 );
my $padding = 2;

my $bufsize  = 2 * $padding + 1;
my $viewline = $padding;
my @buf = ('') x $bufsize;

while ( @buf > 0 ) {
    my $line = shift @lines; # get some data
    shift @buf;              # shorten buffer
    push @buf, $line         # add new line
        if $line;            # if there was one

    print "-->@buf\n" # if there's something to see && it's interesting
        if $buf[$viewline] 
           && grep $buf[$viewline] eq $_, @interesting;
}

##</code><code>##

-->  1 2 3
-->1 2 3 4 5
-->4 5 6 7 8
-->8 9 10 11 12
-->18 19 20