if ($key eq $first .. $key eq $last)
{
    $in = 1;
    push @keys, $key;
}
elsif ($in && $key eq $last)
{
     push @keys, $key;
}
else
{
     $in = 0;
}

##</code><code>##

if ($key eq $first ... $key eq $last)
{
    push @keys, $key;
    if ($key eq $last)
    {
       # do something to process this record in @keys
       # assuming that there will be a new record
       # starting with $first on a subsequent line
    }
}