use strict;
use warnings;

# Get file names, open the input file.
#
my $inputFile = q{spw592046.dat};
my $tempFile  = q{spw592046.tmp};

open my $inputFH, q{<}, $inputFile
   or die qq{open: $inputFile: $!\n};

# Declare, open and close the temporary
# file handle to clobber any file left
# over from a previous run.
#
my $tempFH;
open $tempFH, q{>}, $tempFile
   or die qq{open: $tempFile: $!\n};
close $tempFH
   or die qq{close: $tempFile: $!\n};

# Initialise count of lines written to
# each temporary file. Process input file
# in an infinite loop.
#
my $lineCt = 0;
while (1)
{
    # Read a line from input file. Is it
    # a header line?
    #
    my $line = <$inputFH>;
    if ($line =~ m{^1})
    {
        # Process previous record.
        #
        outputData();

        # Open new temporary file and print
        # line just read to it, counting.
        open $tempFH, q{>}, $tempFile
           or die qq{open: $tempFile: $!\n};
        print $tempFH $line;
        $lineCt = 1;

        # If line just read was at end of
        # file then process one-line record
        # and exit loop.
        #
        if (eof $inputFH)
        {
            outputData();
            last;
        }
    }

    # Not a header line but an ordinary line
    # at the end of file.
    #
    elsif (eof $inputFH)
    {
        # Put line in temporary file, unless
        # it is blank, process the record and
        # exit loop.
        #
        unless ($line =~ m{^\s*$})
        {
            $lineCt ++;
            print $tempFH $line;
        }
        outputData();
        last;
    }

    # Normal line, print to temporary file
    # unless blank.
    #
    else
    {
        next if $line =~ m{^\s*$};
        $lineCt ++;
        print $tempFH $line;
    }
}

sub outputData
{
    # If the line count is +ve we have a
    # temporary file to close so do so.
    #
    if ($lineCt)
    {
        close $tempFH
           or die qq{close: $tempFile: $!\n};
    }

    # We've just read the very first header
    # so there's no record to process yet.
    #
    else
    {
        return;
    }

    # Show how many lines should be in the
    # temporary file.
    #
    print
       qq{\nExpect: $lineCt line},
       $lineCt > 1
       ? qq{s\n}
       : qq{\n};

    # Open temporary file, read and print
    # contents and close again.
    #
    open my $cacheFH, q{<}, $tempFile
       or die qq{open: $tempFile: $!\n};
    while (<$cacheFH>)
    {
        print qq{Cached: $_};
    }
    close $cacheFH
       or die qq{close: $tempFile: $!\n};
}
[download]

100000001
200546Mary
200549#0002897
20055100001
100000003
200546Kathy
200547#0002530
200549#0002897
200552123 Elm Street
100000010
200546Joan
200547#0002530
200549#0002897
  
20055100001
200552681 Ocean Avenue
100000012
[download]

Expect: 4 lines
Cached: 100000001
Cached: 200546Mary
Cached: 200549#0002897
Cached: 20055100001

Expect: 5 lines
Cached: 100000003
Cached: 200546Kathy
Cached: 200547#0002530
Cached: 200549#0002897
Cached: 200552123 Elm Street

Expect: 6 lines
Cached: 100000010
Cached: 200546Joan
Cached: 200547#0002530
Cached: 200549#0002897
Cached: 20055100001
Cached: 200552681 Ocean Avenue

Expect: 1 line
Cached: 100000012
[download]