#!/usr/bin/perl
use warnings;
use strict;

my $dir_to_monitor = 'your/directory/here';
chdir $dir_to_monitor;

# We'll wait for 60 seconds between invocations
my $sleep_time = 60;

# We deem a file 'unchanging' if its size hasn't changed since
# the last time we checked.  Thus, these hashes hold the file
# sizes from our last time through the loop:
my %unchanging_file_sizes;	# Already been sftp'd elsewhere
my %changing_file_sizes;	# Files that are potentially growing

# Holds the state of the loop.  This is set to zero when the file
# named 'stop_sftp_checking' is found.
my $still_running = 1;

while ($still_running) {
	# Check all the files in the directory
	FILE: foreach my $filename (glob '*') {
		# Ignore the file if it hasn't changed
		next FILE
			if (-s $filename == $unchanging_file_sizes{$filename});
		
		# Check files that were changing the last time around to see
		# if they're done changing.
		if (-s $filename == $changing_file_sizes{$filename}) {
			sftp_file($filename);
			delete $changing_file_sizes{$filename};
			$unchanging_file_sizes{$filename} = -s $filename;
			next FILE;
		}
		
		# At this point we can be sure that either the file is new
		# or the file size has changed since we last checked.  Either
		# way, make sure we check it on our next go 'round the loop:
		delete $unchanging_file_sizes{$filename};
		$changing_file_sizes{$filename} = -s $filename;
		
		# Finally, our exit condition.  If we find a file named
		# 'stop_sftp_checking' then quit after handling the last file.
		$still_running = 0 if $filename eq 'stop_sftp_checking';
	}
	
	# Wait and repeat
	sleep $sleep_time;
}