Well, you could do a line for each column. But this is Perl, and There Is More Than One Way To Do It. The documentation for readline_hr() is silent on whether it returns a different hash for each read, but Text::CSV does in fact do this. If you're not nervous about relying on undocumented behavior, and if Text::CSV_XS behaves the same way (Text::CSV is the slower of the two, but does not require a compiler to install), you can simply assign $row to your hash element. But the devil is in the details. When I run what is more or less your code, I find that the data tend to have leading (and sometimes trailing) spaces. So the snippet that assigns the input to the hash becomes

my $base = $row->{'  base addr. (hex)'};
$reqhash{$base} = $row;
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Note that the hard-coded key you look things up in the hash with MUST match the key actually in the hash, both in case and in white space. A lookup on 'Base Addr. (hex)' will find nothing if your file actually contains ' base addr (hex)'. If at some point you are not getting the results you expect, print your data and see if it's what you expect. For example:

use Data::Dumper;
print Dumper( $row );
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For good form the 'use' should be at the top of your code with the other 'use' statements, but if you're just debugging I like to put it with the debugging code, so that when I delete it I delete everything. If you do this more than one place, you can 'use Data::Dumper' more than one place, since module loading is (usually) idempotent. There's probably a slight performance hit, but you are going to remove all the debugging code anyway.

Also, this STILL does not address the problem of non-uniqueness of base addresses. You need to key by base address because that's what you want to sort by. If all you want is a sorted output you could do something like

my $base = "$row->{'  base addr. (hex)'} $row->{' end addr (hex)'}";
$reghash{$base} = $row;
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If you actually want to find and manipulate the individual data, you may have to go to nested hashes -- that is, something like

my $base = $row->{'  base addr. (hex)'};
my $end = $row->{' end addr (hex)'}";
$reghash{$base}{$end} = $row;
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Doing this requires you to know both base address and end address to access a specific row.

You're doing a lot right in your code (three-argument open(), lexical file handles, checking status, 'use strict', 'use warnings'). Good going! These may already have saved you time and grief, and will save you more when you write more Perl.