Hello. You don't need or want a hash. You want an ordinary array. Arrays have indexes, and you don't need while to get all the lines from a file into an array.....

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
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Note that a Perl array starts with index 0, not one.

Are you doing something like this?

my %hash
$hash{$.} = $_ while <>;
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Why not use an array, though? my @lines = <>; The line number is implicit in the array index, and no sorting is needed.

You can grep over the array as much as you wish and still get line numbers (minus 1), my @matched_idxs = grep {$lines[$_] =~ /$re/} 0 .. $#lines; and that array can be used in a slice or a pretty-printing loop.

If you're trying to preserve the order in which lines were read out of a file, and maintain a one-to-one relationship with lines to array elements, I don't see what the problem is. The following snippet is intentionally overly explicit. It could be done in a couple of lines (or less), but I wanted to make it clear what the mechanics are.

my @array;
while ( my $line = <DATA> ) {
    chomp;
    push @array, $line;
}

for( my $idx = 0; $idx <= $#array; $idx++ ) {
    my $line_no = $idx + 1;
    print "Line $line_no = $array[$idx]\n";
}
__DATA__
Here's the first line
Here's the second line
Here is the third line
Here is the forth line
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Just always keep in mind that arrays are zero-indexed.

Hashes don't maintain order, and though it's possible to tie a hash to a class that keeps it sorted, most of the time you shouldn't be trying to enforce order on hashes. On the other hand, if you are using a hash for something where you need to see its keys in sorted order, just sort them.

open INFILE, "<", '/var/tmp/infile' or die "ack! no infile: $!";
@slurp = <INFILE>;
close INFILE;

print "This is line 5:", $slurp[5-1], "\n"; 
# arrays start index at 0 so subtract 
# one from the line number to get the array index.
print "This is line 12:", $slurp[12-1], "\n";
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Sounds like you should investigate Tie::File. It's ideal for this kind of thing.