Perhaps something like this?

...
my $sql = 'select count(*) from replies where reply_id=?';
my $sth = $dbh->prepare($sql) or die $dbh->errstr;
...
my $rc = $sth->execute($q->param('id')) or die $sth->errstr;
my ($count) = $sth->fetchrow_array if ($rc == 1);
...
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