If you repeat this pattern, it tells you one things: You should write it into a subroutine.

sub first_row {
    my ($dbh, $query, @args) = @_
    my $sth = $dbh->prepare($query);
    my $rv = $sth->execute(@args);
    my $first_row = $sth->fetchrow_hashref;
    return ($rv, $first_row);
}

my ($success, $first_result) = first_row($dbh, "SELECT ... ");
# and if you don't want to actually check success
my $second_result = first_row(...);
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(Update: or as ikegami points out, just use $dbh->fetchrow_hashref($query) in the first place.)

(Even more elegant if you don't check for $rv, but instead die on errors, and catch them where appropriate).

If you insist on your current pattern, you can use warnings, but disable single warnings that bother you - read perllexwarn for more details. Or simply put a block around the declarations.