package My::Package;

my $datafile = '/path/to/data.txt';
# each line in file has a numbered key ($key) followed by
# some data.  file is sorted based on incrementing keys 

sub itter_maker {  # return a function that passes back
                   # data only from lines with keys between
                   # $min and $max
  my $min = shift;
  my $max = shift;

  my $done;
  my $fh = new IO::File "$datafile";

  return(
    sub {
      return if $done;
      my (@data,$row,$key);
      while (1) {
        chomp($row = <$fh>) or last;

        ####
        # [SNIPPED] munge $row, set $key and @data
        ####

        next if $key < $min;
        $done = 1 && return if ($key > $max);
        last;
      }
      return ($key,\@data);
    });
}

##</code><code>##

#!/usr/bin/perl
use My::Package;

# only loop through lines with "30 <= $key <= 70"
my $nextrow = itter_maker(30,70);

while (my ($num,$dataref) = $nextrow->()) {
    ### do stuff
}