Best bet would be to stick with the same conventions Perl uses, this way you can maintain a perlish feel to your code making use of 'and' and 'or' so on and so forth. So returning 0, an empty string, or undef are all valid canidates.

Also consider using negative error codes for non fatal errors. I.e all data was present, but a field had some invalid element. This may not be a reason to 'die', but to send it through an extended scrubbing routine.

Just a comment on style, try to segregate lines which do functionally different things. This way it is clear when reading the code that lines grouped together are doing something similar. I have included your code below and spaced it out along what I mean. Also try to not "hardcode" values in your scripts, as this can lead to numerous possibilities for typos. This is just my personal opinion and YMMV, and offered as constructive criticism.

#!/usr/bin/perl 

my $dir   = "/tmp/store";
my $sleep = '1';

opendir DH, $dir or die "Opendir $dir: $!\n";
while (1) {
   for ( grep !/^\.+$/, readdir(DH)  ) {
      my $file = join('/', $dir, $_);
      
# split $_ on _ and return the second element
# this avoids a unnecessary temp array
      my $mobile = ( split /_/ )[1];

# Is the extra whitespace cleanup really necessary?
      $mobile =~ s/(?:^\s+|\s+$)//g; # drop spaces from front and back
      $mobile =~ s/\s+/ /g;          # sanitize the rest 
    
      my $old_sep = $/; # being paranoid, save it for later
      undef $/;
      
      open FH, $file or die "open $file: $!\n";
      chomp( my $data = <FH> );
      close FH;

      $/ = $old_sep; # restore it, in case other funcs expect the defa
+ult
      
# im not sure what the next few lines are doing, as they are
# working on $data or $_, almost randomly.
      my @fields = split;
      my $appcode = $fields[0]; # this is never used again?

      my $message = $data;
      s/^\s+//, s/\s+$//, s/\s\s+/ / for $data; # should this be $mess
+age?

      sendtolive($mobile, $message); # Sends data through socket conne
+ction
    
      unlink("$file"); # remove file once read
   } # END for ( grep readdir(DH) )

   closedir(DH);
   sleep $sleep;
} # END while 1
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