This code looks very strange to me.

if ($key eq $first .. $key eq $last)
{
    $in = 1;
    push @keys, $key;
}
elsif ($in && $key eq $last)
{
     push @keys, $key;
}
else
{
     $in = 0;
}
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One purpose of the "flip-flop" operator is to elminate the need for a flag like $in! The flip-flop operator maintains such a flag internally that is not immediately apparent (and doesn't need to be).

The "if" statement will evaluate to false until $first is seen and remain true up to and including when $last is seen because the flip-flop operator is maintaining the state information that $first has been seen. You do not need to have such a flag yourself.

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
    }
}
[download]

I suggest this classic thread Flipin good, or a total flop? for further study. There is a ".." and a "..." version.