open (INPUT, $ARGV[0]) or die "I couldn't get at input text";
open (SAME_INPUT, $ARGV[0]) or die "I couldn't get at input text";
open (OUTPUT, '>outfile.txt') or die "Can't write to outfile: $!";

while ($line = <INPUT>) {
if (( $line =~ /index_1/ ) and ($line =~ /6/)) {
$index = $.-2;
}
}

while ($line = <SAME_INPUT>) {
if ($. == $index) {
print OUTPUT $line;
}
}