$num_processes = `ps -ef | grep $process_name | grep -v grep | wc -l`;

##</code><code>##

my $STOP_FILE = 'stopfile';

##</code><code>##

my $STOP_FILE = '/home/user/stopfile';

OR
Environment variable should be :-

STOP_FILE_PATH='/home/user';

my $STOP_FILE = "$ENV{STOP_FILE_PATH}/stopfile";