if the 7th element of the array does exist?
You have to be pedantic if you are to be a programmer. The 7th element will always exist unless the array has fewer than 7 elements. But it might be undef or empty or whitespace, etc. so you need to be specific about what your condition to be evaluated is. This example checks for something other than whitespace:
my @line = <DATA>;
print "OMG, line 7 is missing/blank!!!\n" unless $line =~ /\S/;
"It doesn't work" is as useful to a developer as "I'm ill" is to a medic. Try to be specific. SSCCE is best.