#!/usr/bin/perl -w

# Strict
use strict;
use warnings;

# Libraries
use File::Basename;
use FileHandle;

# Main program
$| = 1;

# Create a closure 'psub', which will effectively "tail -f test.txt".
my $psub = tail_closure("test.txt", 0);

# Print the last 10 lines of the file
my $ptext = $psub->(0);
if ($ptext && @$ptext > 0) {
    my @last_10 = (@$ptext > 10)? splice(@$ptext, -10, 10): @$ptext;
    map { print "$_\n" } @last_10;
}

# Monitor any more text written to the file
while (1) {
    my $ptext = $psub->(256);                   # Get any more text
    $ptext and map { print "$_\n" } @$ptext;    # And print it
    select(undef, undef, undef, 0.05);          # Short delay
}

# Subroutines

#
#   Inputs:  $1 ... a file to 'tail' (like "tail -f" in Unix)
#            $2 ... (optional) a flag; if zero, reads from the beginning
#                   of the file; otherwise reads from the end
#
#  Outputs:  $1 ... a pointer to a subroutine to read successive
#                   lines of text from the file.
#
#            Parameters to the closure are:
#
#               Inputs:  $1 ... (optional) maximum number of lines
#                               to read at a time (zero = unlimited)
#
#              Outputs:  $1 ... pointer to the lines read (zero on
#                               error)
#
sub tail_closure {
    my ($fname, $b_from_eof) = @_;
    $b_from_eof ||= 0;
    my $fh = new FileHandle;
    open($fh, "<", $fname) or die "Failure to read file '$fname' ($!)\n";
    $b_from_eof and seek $fh, 0, 2;

    require IO::Select;
    my $select = IO::Select->new();
    $select->add($fh);
    my $text = "";

    my $psub = sub {
        my ($max) = @_;
        $max ||= 0;
        my $count = 0;
        my $plines = [ ];

        while (1) {
            my $c;                              # Next character
            my @ready = $select->can_read(0);   # Ready to read from file?
            @ready or return $plines;           # Not ready - return any lines
            my $nread = sysread $fh, $c, 1;     # Read next character
            defined($nread) or return 0;        # End of file
            ($nread > 0) or return $plines;     # Done reading for now
            if ($c eq "\n") {                   # If newline ...
                push @$plines, $text;           #     Save line of text
                $text = "";                     #     and blank-out text

                # Return the lines when we reach the max
                ($max > 0 && ++$count == $max) and return $plines;
            } else {
                $text .= $c;                    # Add character to text
            }
        }
    };

    return $psub;
}