$_{foo}; #$_ containts a hashref.

##</code><code>##

use strict;
use warnings;
use DBI;
use Data::Dumper;

#connect and execute stored procedure 
#...
push @results, $_{user_id} 
     while ($_ = $sth->fetchrow_hashref());
print Dumper(\@results);

##</code><code>##

$VAR1 = [
          undef,
          undef,
          undef
        ];