Perl hangs when pushing onto array - perl

Question

I'm running the following code to extract a list of IDs from a mysql database and store them in an array. I'm doing this exact sequence of steps in other places in my code, but in this location, something funny happens and Perl just hangs and never returns. Every where else in the code, it works fine. If I run this code as is, it runs fine, but if I uncomment the push() function, it hangs. Is this a bug?
my $result = $db->query($sqlstring)
if ($result->numrows > 0) {
my #list = ();
while (my %row = $result->fetchhash) {
my $studyid = $row{'study_id'} + 0;
print "$studyid\n";
WriteLog("Found study [" . $studyid . "]");
#push(#list,$studyid); # uncomment this to hang it
}
return \#list;
}
EDIT: I tried the data::dumper. But now the freezing occurs in a new location. In the code below, it freezes right after the very last Dumper(#list) statement. (I also switched to DBI from MySQL module, but that had no effect).
my #list = ();
my $result = $dbh->prepare($sqlstring);
$result->execute();
WriteLog($sqlstring);
if ($result->rows > 0) {
while (my $row = $result->fetchrow_hashref()) {
my $studyid = $row->{study_id};
WriteLog("Found study [" . $studyid . "]");
push #list,$studyid;
}
}
print Dumper(#list);
return \#list;

Answer 1

When is this freezing? Is it freezing on the first call to push, or on some subsequent call?
Are you sure you're pushing what you think you're pushing?
What you should do is to use Data::Dumper; and then do a dumper before the push.
use Data::Dumper;
use feature qw(say);
my $result = $db->query($sqlstring)
if ( $result->numrows > 0 ) {
my #list;
while ( my %row = $result->fetchhash ) {
my $studyid = $row{study_id} + 0;
say $studyid;
WriteLog("Found study [" . $studyid . "]");
say "List: " . Dumper #list;
say "Study ID: " . Dumper \$studyid;
push #list, $studyid; # uncomment this to hang it
}
return \#list;
}
Note I'm using say instead of print. Dumper can cause problems if you don't parenthesize it and use print. If you don't want to use say, then you'll need to do this:
print "Study ID: " . Dumper ( \$studyid ) . "\n";
By the way, what do you return if there are no rows?

Answer 2

I was going to mention that we just had this same problem with one of our scripts. The script is being run through Jenkins and the problem was that there wasn't really a problem. We just hadn't seen all of the output up to the point of the program's current state. So it looked like it was hung on a push - it was actually hung legitimately on a sleep() call a few lines later. It was fixed with the auto-flush flag which David W. mentioned. The problem was worsened by the use of Jenkins as an execution environment (vs the command-line debugger). The STDOUT/STDIN pipe opened in the Jenkins context doesn't exist in a command-line shell. So we would never see the issue from the command line - further confusing us. Once I realized that there was a communications pipe involved then things fell into place.