my @lines = <DATA>;
chomp(@lines);

my $line_num  = 3;
my $ary_index = $line_num -1;
printf "Line number %d is '%s'\n", $line_num, $lines[$ary_index];

__DATA__
line 1 index 0
line 2 index 1
line 3 index 2
line 4 index 3
line 5 index 5