You show a sort of tattered remnant of code, so I'm not sure what the best fix is. It looks like you want to do this,

# loop starts upward of here
# $key must be scoped outside the loop
    chomp;
    if (/^\d+\.\d+\.\d+\.\d+$/) {
        next unless $key;
        push @{$CoreRegR{$key}}, $_;
        # Alt. replace the prev 2 lines with
        # push @{$CoreRegR{ $key || 'unnamed host' }}, $_;
    }
    else {
        $key = $_
    }
# ...
[download]

It is simpler to push things straight into the HoA element when they are encountered than to build a named array and shove it in when the next key is found. As you observed, that loses the last set of addresses.

After Compline,
Zaxo