my $tempfile= '/tmp/sql_output';
my $sql = "select * from users";
open (COMMAND, "| /opt/sql >$tempfile" ) || die $!;
print (COMMAND "connect credentials")
print (COMMAND "$sql\n");

open my $results, '<', $tempfile
    or die "Couldn't read '$tempfile': $!";
while( <$results> ) {
    print $_;
};